Embodiments of this application relate to the communications field, and disclose an inter-communications-system moving method, a device, and a system, to implement moving of UE between communications systems and delete a session incapable of being transferred. The inter-communications-system moving method includes: determining, by user equipment UE, whether a first session exists in a first communications system, where the first session is capable of being transferred from the first communications system to a second communications system; and sending, by the UE, a first message to a first core network device of the second communications system when determining that the first session exists, where the first message is used to establish a second session in the second communications system, and the second session corresponds to the first session.