This application relates to a method and a communication apparatus. The method includes: A terminal device enters a first mode when the terminal device completes registration with a non-terrestrial mobile communication network NTN, where the terminal device does not need to initiate, in the first mode, periodic registration with a first core network device in the NTN. That the terminal device completes the registration with the NTN may be understood as that the terminal device sends, to the first core network device, a first message used for requesting registration, and receives, from the first core network device, a second message indicating that a registration request is accepted.