Finally find the correct way to reset a form and to load default value.

static/plugin_dbui/src/appform.js

@@ -170,7 +170,7 @@ App.form.FormPanel = Ext.extend(Ext.form.FormPanel, {
         var form = this.getForm();
         
         form.items.each(function (field) {
-            field.originalValue = "";
+            field.originalValue = field.initialConfig.value;
         });
         
         form.reset();