diff --git a/lib/download/npm.js b/lib/download/npm.js
index 71414cad..62362e93 100644
--- a/lib/download/npm.js
+++ b/lib/download/npm.js
@@ -529,7 +529,6 @@ const defaultExtensions = toMap([
   '.markdown',
   '.md',
   '.mkd',
-  '.ts',
   '.jst',
   '.coffee',
   '.tgz',
diff --git a/test/install-enable-prune.test.js b/test/install-enable-prune.test.js
index 9d6a15be..9b91b873 100644
--- a/test/install-enable-prune.test.js
+++ b/test/install-enable-prune.test.js
@@ -38,6 +38,8 @@ describe('test/install-enable-prune.test.js', () => {
         .end();
       const exists = await fs.exists(path.join(cwd, 'node_modules/egg/README.md'));
       assert(!exists);
+      // should keep ts file
+      assert(await fs.exists(path.join(cwd, 'node_modules/egg/index.d.ts')));
     });
   });
 });