diff --git a/apps/emqx_connector/src/emqx_connector_http.erl b/apps/emqx_connector/src/emqx_connector_http.erl
index 2b9bd48aa..21c06284d 100644
--- a/apps/emqx_connector/src/emqx_connector_http.erl
+++ b/apps/emqx_connector/src/emqx_connector_http.erl
@@ -266,11 +266,16 @@ process_request(#{
         } = Conf, Msg) ->
     Conf#{ method => make_method(emqx_plugin_libs_rule:proc_tmpl(MethodTks, Msg))
          , path => emqx_plugin_libs_rule:proc_tmpl(PathTks, Msg)
-         , body => emqx_plugin_libs_rule:proc_tmpl(BodyTks, Msg)
+         , body => process_request_body(BodyTks, Msg)
          , headers => maps:to_list(proc_headers(HeadersTks, Msg))
          , request_timeout => ReqTimeout
          }.
 
+process_request_body([], Msg) ->
+    emqx_json:encode(Msg);
+process_request_body(BodyTks, Msg) ->
+    emqx_plugin_libs_rule:proc_tmpl(BodyTks, Msg).
+
 proc_headers(HeaderTks, Msg) ->
     maps:fold(fun(K, V, Acc) ->
             Acc#{emqx_plugin_libs_rule:proc_tmpl(K, Msg) =>
diff --git a/apps/emqx_connector/src/mqtt/emqx_connector_mqtt_msg.erl b/apps/emqx_connector/src/mqtt/emqx_connector_mqtt_msg.erl
index eb483dcc5..1357037ee 100644
--- a/apps/emqx_connector/src/mqtt/emqx_connector_mqtt_msg.erl
+++ b/apps/emqx_connector/src/mqtt/emqx_connector_mqtt_msg.erl
@@ -66,7 +66,7 @@ to_remote_msg(#message{flags = Flags0} = Msg, Vars) ->
 to_remote_msg(MapMsg, #{remote_topic := TopicToken, payload := PayloadToken,
         remote_qos := QoSToken, retain := RetainToken, mountpoint := Mountpoint}) when is_map(MapMsg) ->
     Topic = replace_vars_in_str(TopicToken, MapMsg),
-    Payload = replace_vars_in_str(PayloadToken, MapMsg),
+    Payload = process_payload(PayloadToken, MapMsg),
     QoS = replace_simple_var(QoSToken, MapMsg),
     Retain = replace_simple_var(RetainToken, MapMsg),
     #mqtt_msg{qos = QoS,
@@ -82,13 +82,18 @@ to_broker_msg(#{dup := Dup, properties := Props} = MapMsg,
             #{local_topic := TopicToken, payload := PayloadToken,
               local_qos := QoSToken, retain := RetainToken, mountpoint := Mountpoint}) ->
     Topic = replace_vars_in_str(TopicToken, MapMsg),
-    Payload = replace_vars_in_str(PayloadToken, MapMsg),
+    Payload = process_payload(PayloadToken, MapMsg),
     QoS = replace_simple_var(QoSToken, MapMsg),
     Retain = replace_simple_var(RetainToken, MapMsg),
     set_headers(Props,
         emqx_message:set_flags(#{dup => Dup, retain => Retain},
             emqx_message:make(bridge, QoS, topic(Mountpoint, Topic), Payload))).
 
+process_payload([], Msg) ->
+    emqx_json:encode(Msg);
+process_payload(Tks, Msg) ->
+    replace_vars_in_str(Tks, Msg).
+
 %% Replace a string contains vars to another string in which the placeholders are replace by the
 %% corresponding values. For example, given "a: ${var}", if the var=1, the result string will be:
 %% "a: 1".