A communication method and related device are disclosed. In at least one embodiment, a reference message authentication code is obtained by a terminal device based on a first message authentication code and a second message authentication code. The first message authentication code is used by an AMF to check a service request message, and the second message authentication code is used by an SMF to check the service request message. and sending, by the terminal device, first information and the service request message are sent by the terminal device to the AMF. The first information is used to instruct the AMF to send the service request message to the SMF. The service request message includes second information. The second information includes the reference message authentication code. The SMF can perform a security check on the service request message to ensure security of a communications network.