A method and a system for accounting supporting Quality of Service (QoS) change during a session, and an AAA client and an accounting processing unit are disclosed. The method includes: detecting by an AAA client a QoS change notification of a session; obtaining by the AAA client the accounting information of the session; and sending the accounting information to an accounting processing unit of a AAA server; implementing by the accounting processing unit an accounting operation for the user of the session according to the accounting information. The system includes the AAA client and the accounting processing unit of the AAA server. By the embodiment of this invention, the accounting for a radio access network user may be more accurate and reasonable.