The invention disclose a method for verifying the validity of a user, making full use of a TID as the bridge for establishing confidence between a NAF and a user equipment, and the BSF assigning a term of validity for the TID, thereby extending the function of the TID, enabling the NAF to verify the term of validity for using the TID, and accordingly, achieving a further verification of the validity to the user. By using the method of the invention, it is possible to avoid the situation in which one TID is permanently valid for one or more NAFs, enhance the system security, decrease the risks caused by the theft of users' TID and corresponding secret keys, and at the same time, implement TID management by the NAF. In addition, a combination of the method with billing system makes it easy to implement the function of charging a user.