diff --git a/.changeset/flat-planes-laugh.md b/.changeset/flat-planes-laugh.md
new file mode 100644
index 0000000000..e508b7ffa6
--- /dev/null
+++ b/.changeset/flat-planes-laugh.md
@@ -0,0 +1,5 @@
+---
+"@lynx-js/react-rsbuild-plugin": patch
+---
+
+Avoid entry IIFE in `main-thread.js`
diff --git a/packages/rspeedy/plugin-react/src/loaders/ignore-css-loader.ts b/packages/rspeedy/plugin-react/src/loaders/ignore-css-loader.ts
index 10d7420bd7..b284c80a88 100644
--- a/packages/rspeedy/plugin-react/src/loaders/ignore-css-loader.ts
+++ b/packages/rspeedy/plugin-react/src/loaders/ignore-css-loader.ts
@@ -13,7 +13,9 @@ export default function ignoreCssLoader(
   // It is not a CSS Modules file because exportOnlyLocals is enabled,
   // so we don't need to preserve it.
   if (source.includes('___CSS_LOADER_EXPORT___')) {
-    return ''
+    // Return an ESM to make sure the module strict.
+    // See: https://github.com/webpack/webpack/discussions/18367#discussion-6580398
+    return 'export {}'
   }
 
   // Preserve css modules export for background layer.