A key generation method includes a user plane network function and a terminal device obtain key update information sent by each other. The user plane network function updates, by using the obtained key update information, a sub-key derived from a permanent key, to obtain a new protection key. The terminal device updates, by using the obtained key update information, a sub-key derived from the permanent key, to obtain a new protection key. The terminal device and the user plane network function perform, by using the new protection key, security protection on user plane data transmitted between the terminal device and the user plane network function.