A method for selecting an accounting mode, a method for switching an accounting mode, and a device therefor. The method for switching an accounting mode includes: notifying a prepaid accounting server to establish a session; assigning an accounting quota of the session to the prepaid client; notifying the prepaid accounting server that the accounting quota of the session is depleted, and notifying the prepaid client to stop the prepaid accounting; keeping the session established and notifying an AAA server to start a postpaid accounting.