A method and a system for single sign-on are provided by the present invention, wherein the method comprises: a terminal sending an authentication request carrying a user identity identification to an RP and the RP redirecting the authentication request to an authentication center; the authentication center authenticating the terminal by means of SIP Digest and redirecting the authentication result to the RP via the terminal; and the RP providing services for the terminal according to an authentication result. By the present invention, the resources required by the operators to deploy GBAs are reduced, at the same time the requirement of non-UICC terminals accessing IMS network can be met, and the relevant application services of the IMS network can be accessed by means of SSO.