A Session Initiation Protocol (SIP) authentication method, sends a request message without authentication information to a server end from a client to request for access; sends back a response message, which contains authentication exchange information and DH authentication response information of the server end when the server end receives the request message; authenticates the received response message by the client and sending a request message, which contains authentication information of the client, to the server end after the authentication is passed; authenticates a user according to the received request message by the server end, and sends back a response message which contains the authentication information of the server end; authenticates the legality of the server end by the user according to the received response message, which contains the authentication information of the server end.