diff --git a/src/lib/components/JsonPreview.svelte b/src/lib/components/JsonPreview.svelte
index fdd97aa..fd2e198 100644
--- a/src/lib/components/JsonPreview.svelte
+++ b/src/lib/components/JsonPreview.svelte
@@ -1,13 +1,39 @@
 <script lang="ts">
-	let { json }: { json: Record<string, unknown>[] } = $props();
+	let { json, onupdate }: {
+		json: Record<string, unknown>[];
+		onupdate?: (edited: string) => void;
+	} = $props();
 
-	let collapsed = $state(false);
+	const MAX_PREVIEW_CHARS = 2000;
+	const MAX_PREVIEW_LINES = 50;
+
+	let editing = $state(false);
+	let editText = $state('');
+	let parseError = $state('');
 
 	const jsonString = $derived(JSON.stringify(json, null, 2));
 
-	/**
-	 * Simple JSON syntax highlighting — light theme.
-	 */
+	// Truncation for preview
+	const truncated = $derived.by(() => {
+		const lines = jsonString.split('\n');
+		if (jsonString.length <= MAX_PREVIEW_CHARS && lines.length <= MAX_PREVIEW_LINES) {
+			return { text: jsonString, isTruncated: false, totalChars: jsonString.length, totalLines: lines.length };
+		}
+		// Truncate by line first, then by char
+		let preview = lines.slice(0, MAX_PREVIEW_LINES).join('\n');
+		if (preview.length > MAX_PREVIEW_CHARS) {
+			preview = preview.slice(0, MAX_PREVIEW_CHARS);
+		}
+		return { text: preview, isTruncated: true, totalChars: jsonString.length, totalLines: lines.length };
+	});
+
+	// Sync from upstream when not editing
+	$effect(() => {
+		if (!editing) {
+			editText = jsonString;
+		}
+	});
+
 	function highlight(str: string): string {
 		return str
 			.replace(/&/g, '&amp;')
@@ -19,6 +45,29 @@
 			.replace(/:\s*(true|false)/g, ': <span class="json-bool">$1</span>')
 			.replace(/:\s*(null)/g, ': <span class="json-null">$1</span>');
 	}
+
+	function startEdit() {
+		editText = jsonString;
+		editing = true;
+		parseError = '';
+	}
+
+	function cancelEdit() {
+		editing = false;
+		parseError = '';
+		editText = jsonString;
+	}
+
+	function applyEdit() {
+		try {
+			JSON.parse(editText);
+			parseError = '';
+			editing = false;
+			onupdate?.(editText);
+		} catch {
+			parseError = 'JSON 格式错误，请检查语法';
+		}
+	}
 </script>
 
 <div class="flex h-full flex-col overflow-hidden">
@@ -27,21 +76,44 @@
 			JSON 预览
 			<span class="ml-2 text-xs font-normal text-gray-400">{json.length} 条记录</span>
 		</h3>
-		<div class="flex items-center gap-2">
-			<button
-				onclick={() => (collapsed = !collapsed)}
-				class="rounded px-2 py-1 text-xs text-gray-500 hover:bg-gray-200 cursor-pointer"
-			>
-				{collapsed ? '展开' : '折叠'}
-			</button>
+		<div class="flex items-center gap-1.5">
+			{#if editing}
+				<button onclick={applyEdit} class="rounded bg-blue-600 px-2 py-1 text-xs text-white hover:bg-blue-700 cursor-pointer">
+					应用
+				</button>
+				<button onclick={cancelEdit} class="rounded border border-gray-300 px-2 py-1 text-xs text-gray-500 hover:bg-gray-200 cursor-pointer">
+					取消
+				</button>
+			{:else}
+				<button onclick={startEdit} class="rounded border border-gray-300 px-2 py-1 text-xs text-gray-500 hover:bg-gray-200 cursor-pointer">
+					编辑
+				</button>
+			{/if}
 		</div>
 	</div>
 
-	<div class="json-preview flex-1 overflow-auto p-4">
-		{#if collapsed}
-			<pre class="font-mono text-xs leading-relaxed text-gray-600">{JSON.stringify(json)}</pre>
+	{#if parseError}
+		<div class="flex-shrink-0 bg-red-50 px-4 py-1.5 text-xs text-red-600">{parseError}</div>
+	{/if}
+
+	<div class="json-preview flex-1 overflow-hidden">
+		{#if editing}
+			<!-- Plain textarea for editing — no overlay highlighting to avoid lag -->
+			<textarea
+				bind:value={editText}
+				oninput={() => { parseError = ''; }}
+				spellcheck="false"
+				class="json-textarea h-full w-full resize-none overflow-auto border-0 bg-transparent font-mono text-xs leading-relaxed text-gray-700 outline-none"
+			></textarea>
 		{:else}
-			<pre class="font-mono text-xs leading-relaxed text-gray-500">{@html highlight(jsonString)}</pre>
+			<div class="h-full overflow-auto">
+				<pre class="json-content whitespace-pre font-mono text-xs leading-relaxed text-gray-500">{@html highlight(truncated.text)}</pre>
+				{#if truncated.isTruncated}
+					<div class="truncation-notice sticky bottom-0 border-t border-amber-200 bg-amber-50 px-4 py-2 text-xs text-amber-700">
+						&#x26A0;&#xFE0F; 预览已截断以提升性能（显示 {Math.min(MAX_PREVIEW_CHARS, truncated.text.length).toLocaleString()}/{truncated.totalChars.toLocaleString()} 字符，{Math.min(MAX_PREVIEW_LINES, truncated.totalLines)}/{truncated.totalLines.toLocaleString()} 行）。请点击下载或复制查看完整数据。
+					</div>
+				{/if}
+			</div>
 		{/if}
 	</div>
 </div>
@@ -49,6 +121,16 @@
 <style>
 	.json-preview {
 		background: #fafbfc;
+		content-visibility: auto;
+	}
+	.json-content {
+		padding: 16px;
+		margin: 0;
+		content-visibility: auto;
+	}
+	.json-textarea {
+		padding: 16px;
+		tab-size: 2;
 	}
 	:global(.json-key) {
 		color: #24292e;
diff --git a/src/lib/excel.ts b/src/lib/excel.ts
index 0d73c10..c593ede 100644
--- a/src/lib/excel.ts
+++ b/src/lib/excel.ts
@@ -39,6 +39,18 @@ export function parseSheet(workbook: XLSX.WorkBook, sheetName: string): ParsedEx
 		throw new Error('工作表中没有数据');
 	}
 
-	const headers = Object.keys(jsonData[0]);
-	return { headers, rows: jsonData, sheetNames: workbook.SheetNames };
+	const allHeaders = Object.keys(jsonData[0]);
+	// Filter out SheetJS placeholder headers for empty columns (__EMPTY, __EMPTY_1, etc.)
+	const headers = allHeaders.filter((h) => !/^__EMPTY(_\d+)?$/.test(h));
+
+	// Strip empty-column keys from rows
+	const rows = headers.length < allHeaders.length
+		? jsonData.map((row) => {
+				const cleaned: Record<string, unknown> = {};
+				for (const h of headers) cleaned[h] = row[h];
+				return cleaned;
+			})
+		: jsonData;
+
+	return { headers, rows, sheetNames: workbook.SheetNames };
 }
diff --git a/src/routes/+page.svelte b/src/routes/+page.svelte
index 71563ce..4285068 100644
--- a/src/routes/+page.svelte
+++ b/src/routes/+page.svelte
@@ -14,6 +14,34 @@
 	let isDragOver = $state(false);
 	let copySuccess = $state(false);
 
+	// Split panel
+	let splitPercent = $state(50);
+	let jsonCollapsed = $state(false);
+	let isDraggingSplit = $state(false);
+	let containerEl = $state<HTMLDivElement>();
+
+	function onSplitPointerDown(e: PointerEvent) {
+		e.preventDefault();
+		isDraggingSplit = true;
+		(e.target as HTMLElement).setPointerCapture(e.pointerId);
+	}
+
+	function onSplitPointerMove(e: PointerEvent) {
+		if (!isDraggingSplit || !containerEl) return;
+		const rect = containerEl.getBoundingClientRect();
+		const pct = ((e.clientX - rect.left) / rect.width) * 100;
+		splitPercent = Math.max(20, Math.min(80, pct));
+	}
+
+	function onSplitPointerUp() {
+		isDraggingSplit = false;
+	}
+
+	function toggleJsonPanel() {
+		jsonCollapsed = !jsonCollapsed;
+		if (!jsonCollapsed && splitPercent > 80) splitPercent = 50;
+	}
+
 	// Derived
 	const hasData = $derived(headers.length > 0 && rows.length > 0);
 
@@ -34,7 +62,20 @@
 		}, 150);
 	});
 
-	const jsonString = $derived(JSON.stringify(convertedJson, null, 2));
+	// Manual JSON editing — overrides converted output until next mapping/data change
+	let manualJson = $state<string | null>(null);
+
+	const jsonString = $derived(manualJson ?? JSON.stringify(convertedJson, null, 2));
+
+	function onJsonEdited(edited: string) {
+		manualJson = edited;
+	}
+
+	// Clear manual override when conversion changes
+	$effect(() => {
+		convertedJson;
+		manualJson = null;
+	});
 
 	// File handling
 	async function handleFile(file: File) {
@@ -239,15 +280,56 @@
 
 	<!-- Main content -->
 	{#if hasData}
-		<div class="flex flex-1 overflow-hidden">
+		<!-- svelte-ignore a11y_no_static_element_interactions -->
+		<div
+			bind:this={containerEl}
+			class="relative flex flex-1 overflow-hidden"
+			onpointermove={onSplitPointerMove}
+			onpointerup={onSplitPointerUp}
+		>
 			<!-- Left: Excel Table -->
-			<div class="w-1/2 overflow-hidden border-r border-gray-300">
+			<div class="overflow-hidden" style="width: {jsonCollapsed ? '100%' : `${splitPercent}%`}">
 				<ExcelTable {headers} {rows} bind:mappings />
 			</div>
-			<!-- Right: JSON Preview -->
-			<div class="w-1/2 overflow-hidden">
-				<JsonPreview json={convertedJson} />
-			</div>
+
+			{#if !jsonCollapsed}
+				<!-- Drag handle -->
+				<!-- svelte-ignore a11y_no_static_element_interactions -->
+				<div
+					class="group flex w-2 flex-shrink-0 cursor-col-resize items-center justify-center bg-gray-200 hover:bg-blue-300 active:bg-blue-400 transition-colors"
+					onpointerdown={onSplitPointerDown}
+				>
+					<div class="h-8 w-0.5 rounded-full bg-gray-400 group-hover:bg-blue-500"></div>
+				</div>
+
+				<!-- Right: JSON Preview -->
+				<div class="relative flex-1 overflow-hidden">
+					<button
+						onclick={toggleJsonPanel}
+						aria-label="收起 JSON 面板"
+						class="absolute top-2 right-2 z-10 rounded bg-white/80 p-1 text-gray-400 shadow hover:bg-white hover:text-gray-600 cursor-pointer"
+					>
+						<svg class="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+							<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M13 5l7 7-7 7M5 5l7 7-7 7" />
+						</svg>
+					</button>
+					<JsonPreview json={convertedJson} onupdate={onJsonEdited} />
+				</div>
+			{:else}
+				<!-- Collapsed JSON mini-panel -->
+				<div class="flex w-10 flex-shrink-0 flex-col items-center border-l border-gray-200 bg-gray-50 py-3">
+					<button
+						onclick={toggleJsonPanel}
+						aria-label="展开 JSON 面板"
+						class="rounded p-1 text-gray-400 hover:bg-gray-200 hover:text-gray-600 cursor-pointer"
+					>
+						<svg class="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+							<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M11 19l-7-7 7-7M19 19l-7-7 7-7" />
+						</svg>
+					</button>
+					<span class="mt-2 text-xs text-gray-400" style="writing-mode: vertical-rl">JSON</span>
+				</div>
+			{/if}
 		</div>
 	{:else}
 		<!-- Empty state / drop zone -->