Safe context negotiation method and system in authentication process

The invention discloses a method and a system for negotiating security context during the re-authentication and key agreement under the state that the security context is activated, which are used for solving the problem that UE fails to perform the AKA again under the state that NAS security context is activated. An MME performs encryption and integrity protection on an authentication request by using the current security context to ensure that an authentication request message can successfully pass the processing of the UE on a NAS message; and the method and the system trigger the MME and the UE to stop using the current NAS security context through the authentication request message, ensure that the UE and the MME can perform plaintext processing on subsequent authentication requests and the second AKA which possibly appears subsequently, and save the processing time of the UE and the MME.