diff --git a/src/rootserver/ddl_task/ob_ddl_scheduler.cpp b/src/rootserver/ddl_task/ob_ddl_scheduler.cpp
index 055dcbd8f6..e112c46ed6 100644
--- a/src/rootserver/ddl_task/ob_ddl_scheduler.cpp
+++ b/src/rootserver/ddl_task/ob_ddl_scheduler.cpp
@@ -417,8 +417,6 @@ int ObDDLTaskHeartBeatMananger::get_inactive_ddl_task_ids(ObArray<int64_t>& remo
   return ret;
 }
 
-ObPrepareAlterTableArgParam::ObPrepareAlterTableArgParam() : session_id_(OB_INVALID_ID) {}
-
 int ObPrepareAlterTableArgParam::init(const uint64_t session_id,
                                       const ObSQLMode &sql_mode,
                                       const ObString &ddl_stmt_str,
@@ -458,9 +456,8 @@ int ObPrepareAlterTableArgParam::set_nls_formats(const common::ObString *nls_for
     ret = OB_INVALID_ARGUMENT;
   } else {
     char *tmp_ptr[ObNLSFormatEnum::NLS_MAX] = {};
-    common::ObArenaAllocator allocator;
     for (int64_t i = 0; OB_SUCC(ret) && i < ObNLSFormatEnum::NLS_MAX; ++i) {
-      if (OB_ISNULL(tmp_ptr[i] = (char *)allocator.alloc(nls_formats[i].length()))) {
+      if (OB_ISNULL(tmp_ptr[i] = (char *)allocator_.alloc(nls_formats[i].length()))) {
         ret = OB_ALLOCATE_MEMORY_FAILED;
         SHARE_LOG(ERROR, "failed to alloc memory!", "size", nls_formats[i].length(), K(ret));
       } else {
@@ -470,7 +467,7 @@ int ObPrepareAlterTableArgParam::set_nls_formats(const common::ObString *nls_for
     }
     if (OB_FAIL(ret)) {
       for (int64_t i = 0; i < ObNLSFormatEnum::NLS_MAX; ++i) {
-        allocator.free(tmp_ptr[i]);
+        allocator_.free(tmp_ptr[i]);
       }
     }
   }
diff --git a/src/rootserver/ddl_task/ob_ddl_scheduler.h b/src/rootserver/ddl_task/ob_ddl_scheduler.h
index f2d1fa45de..6064ad7f2f 100644
--- a/src/rootserver/ddl_task/ob_ddl_scheduler.h
+++ b/src/rootserver/ddl_task/ob_ddl_scheduler.h
@@ -104,7 +104,13 @@ private:
 struct ObPrepareAlterTableArgParam final
 {
 public:
-  ObPrepareAlterTableArgParam();
+  ObPrepareAlterTableArgParam() :
+    session_id_(common::OB_INVALID_ID),
+    sql_mode_(0),
+    tz_info_wrap_(),
+    allocator_(lib::ObLabel("PrepAlterTblArg")),
+    nls_formats_{}
+  {}
   ~ObPrepareAlterTableArgParam() = default;
   int init(const uint64_t session_id,
           const ObSQLMode &sql_mode,
@@ -140,6 +146,7 @@ public:
   common::ObString target_database_name_;
   common::ObTimeZoneInfo tz_info_;
   common::ObTimeZoneInfoWrap tz_info_wrap_;
+  common::ObArenaAllocator allocator_;
   common::ObString nls_formats_[common::ObNLSFormatEnum::NLS_MAX];
 };