diff --git a/cpp/CHANGELOG.md b/cpp/CHANGELOG.md
index 74b6703a2822e523abd06844d4dd56aba40abc09..06511ae01b945e3c8ff48b6f0579e7a121cd4ea9 100644
--- a/cpp/CHANGELOG.md
+++ b/cpp/CHANGELOG.md
@@ -26,6 +26,7 @@ Please mark all change in change log and use the ticket from JIRA.
 - MS-467 - mysql db test failed
 - MS-470 - Drop index success, which table not created
 - MS-471 - code coverage run failed
+- MS-492 - Drop index failed if index have been created with index_type: FLAT
 - MS-493 - Knowhere unittest crash
 
 ## Improvement
diff --git a/cpp/src/server/grpc_impl/GrpcRequestTask.cpp b/cpp/src/server/grpc_impl/GrpcRequestTask.cpp
index e0529c1999b080056d6bf29c4de87ccee12246b2..4bbc15f87357d61820f8f038b1240c5d9a6d21c2 100644
--- a/cpp/src/server/grpc_impl/GrpcRequestTask.cpp
+++ b/cpp/src/server/grpc_impl/GrpcRequestTask.cpp
@@ -955,15 +955,8 @@ DropIndexTask::OnExecute() {
             return SetError(res, "Invalid table name: " + table_name_);
         }
 
-        //step 2:check index existence
-        engine::TableIndex index;
-        engine::Status stat = DBWrapper::DB()->DescribeIndex(table_name_, index);
-        if (index.engine_type_ == 1) {
-            return SetError(SERVER_UNEXPECTED_ERROR, "index not existed");
-        }
-
-        //step 3: check table existence
-        stat = DBWrapper::DB()->DropIndex(table_name_);
+        //step 2: check table existence
+        auto stat = DBWrapper::DB()->DropIndex(table_name_);
         if (!stat.ok()) {
             return SetError(DB_META_TRANSACTION_FAILED, stat.ToString());
         }