A service continuity implementation method, a related device, and a system, where a mobility management device may select a 5G gateway for a 5G user equipment handed over to a 4G network, and establish a session with the 5G gateway. The 5G gateway may send a 5G-related parameter to the user equipment in advance. Therefore, after the user equipment moves from the 4G network to a 5G network, service continuity is maintained, that is, a service in the 4G network continues to be maintained in the 5G network.