diff --git a/apps/emqx_connector/src/emqx_connector_mongo.erl b/apps/emqx_connector/src/emqx_connector_mongo.erl
index 391075800..bd02ccc02 100644
--- a/apps/emqx_connector/src/emqx_connector_mongo.erl
+++ b/apps/emqx_connector/src/emqx_connector_mongo.erl
@@ -77,9 +77,7 @@ fields(sharded) ->
     ] ++ mongo_fields();
 fields(topology) ->
     [ {pool_size, fun emqx_connector_schema_lib:pool_size/1}
-        %% TODO: what is this
-        %% 'max_overflow'
-    , {max_overflow, fun emqx_connector_schema_lib:pool_size/1}
+    , {max_overflow, fun max_overflow/1}
     , {overflow_ttl, fun duration/1}
     , {overflow_check_period, fun duration/1}
     , {local_threshold_ms, fun duration/1}
@@ -330,6 +328,11 @@ duration(desc) -> "Time interval, such as timeout or TTL.";
 duration(required) -> false;
 duration(_) -> undefined.
 
+max_overflow(type) -> non_neg_integer();
+max_overflow(desc) -> "Max Overflow.";
+max_overflow(default) -> 0;
+max_overflow(_) -> undefined.
+
 replica_set_name(type) -> binary();
 replica_set_name(desc) -> "Name of the replica set.";
 replica_set_name(required) -> false;