A method of authentication in an IP Multimedia Subsystem (IMS) is provided. After receiving a Register message from a User Equipment (UE), a Proxy-Call Session Control Function (P-CSCF) locates a Connection Location Function (CLF) according to information contained in the Register message and a pre-configured corresponding relationship between the information contained in the Register message and the CLF. The P-CSCF obtains a query result by querying the CLF about attachment information of the UE in an access network, and sends the Register message carrying the query result to an Interrogating-Call Session Control Function (I-CSCF). The I-CSCF forwards the Register message carrying the query result to a Service-Call Session Control Function (S-CSCF). The S-CSCF authenticates the UE according to an authentication mechanism obtained from a User Profile Service Function (UPSF) or a Home Subscriber Server (HSS), and sends an authentication result to the UE.