A service continuity implementation method, a related device, and a system, where when a user equipment is attached to a 2/3G network, a serving node device determines that the user equipment has a 5G capability or a user corresponding to the user equipment is a 5G user. Then, the serving node device selects a 5G gateway for the user equipment, and establishes a session with the 5G gateway. In this way, when the user equipment subsequently moves to a 5G network, the same 5G gateway serves the user equipment, and an address of the user may remain unchanged when the user equipment moves from the 2/3G network to the 5G network, thereby implementing service continuity of the user.