diff --git a/cpp/src/scheduler/task/SearchTask.cpp b/cpp/src/scheduler/task/SearchTask.cpp
index 937936dd313d42895fd4c8942cd68302c5acd006..63aa4371e9a55dcb2842bc31c41f65462dba29d1 100644
--- a/cpp/src/scheduler/task/SearchTask.cpp
+++ b/cpp/src/scheduler/task/SearchTask.cpp
@@ -269,6 +269,7 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
         return Status::OK();
     }
 
+    merge_mutex_.lock();
     if (distance_target.empty()) {
         distance_target.swap(distance_src);
         return Status::OK();
@@ -328,6 +329,7 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
     }
 
     distance_target.swap(distance_merged);
+    merge_mutex_.unlock();
 
     return Status::OK();
 }
diff --git a/cpp/src/scheduler/task/SearchTask.h b/cpp/src/scheduler/task/SearchTask.h
index 2370e26361b5e4ac4fb11e71263fb5b2c9a705f5..6adda7c1fb118b3b6ac4a4ab7a0a78c2b869ed03 100644
--- a/cpp/src/scheduler/task/SearchTask.h
+++ b/cpp/src/scheduler/task/SearchTask.h
@@ -48,6 +48,8 @@ public:
     int index_type_ = 0;
     ExecutionEnginePtr index_engine_ = nullptr;
     bool metric_l2 = true;
+
+    static std::mutex merge_mutex_;
 };
 
 }