A method and a system for authenticating a mobile terminal, and the method comprises: the mobile terminal using a pre-set password P to decrypt a key A stored in a subscriber information file to obtain a key Ki which is used in authentication; in a process of authentication, the mobile terminal using the key Ki and a random number sent by a network side to generate encrypted authentication response data, and sending the encrypted authentication response data to the network side; and the network side verifying the encrypted authentication response data sent by the mobile terminal, and returning an authentication success acknowledgement to the mobile terminal after the verification succeeds; wherein the subscriber information file is stored in a local memory of the mobile terminal, or a mobile memory device connected with the mobile terminal.