A method of authentication in an IMS includes: after receiving a Register message from a UE, locating, by a P-CSCF, a CLF according to information contained in the Register message and a pre-configured relationship; querying, by the P-CSCF, the CLF about NASS attachment information of the UE to obtain a query result, and sending the Register message carrying the query result to an I-CSCF; forwarding, by the I-CSCF, the Register message carrying the query result to a S-CSCF assigned by a UPSF or the HSS; authenticating the UE and sending an authentication result to the UE by the S-CSCF. In embodiments of the present invention, the UPSF or the HSS in the service layer determines the authentication mechanism of the user, and the S-CSCF implements the authentication, which is more reasonable. Embodiments of the present invention also provide combinations of NBA with other authentication mechanisms, thereby guarantees the authentication of the user after the NBA authentication fails.