help me about form validation callback function

[eluser]WanWizard[/eluser]
Form validation looks for the callback method only in the loaded controller, so in your case $this->CI->_username_exists() must exist for the callback to function.

You have a few options:

If this validation code is specific to this controller, put the callback code back into your controller.
If you want to reuse the validation code in a few other controllers, put the code in a library or model, then create a callback method in your controller that calls the model/library method.
If you want to use the validation code everywhere, extend the form validation library and put the method in your extension, so you can use it like any other rule (without the callback prefix).