The invention discloses a security authentication method, related equipment and a communication system. The communication system comprises MN, CN, UAG and GRIDS systems. The method comprises: the UAGreceiving a BU message sent by the MN and a signature of the MN, and the signature of the MN being obtained by calculating the BU message by using a private key of the MN; calculating the BU message,the signature of the MN and the identifier of the UAG by using the private key of the UAG to obtain the signature of the UAG; sending the BU message, the signature of the MN and the signature of the UAG to the CN; receiving a BU response message and a signature of the CN sent by the CN, wherein the BU response message comprises a result obtained by verifying the signatures of the UAG and the MN; and sending the BU response message and the signature of the CN to the MN. By implementing the method and the device, the security authentication problem after the ID is added to the network layer canbe solved, rapid authentication and network switching based on the ID are realized, the authentication process is simplified, the authentication efficiency is improved, and the switching time delay isreduced.