A RAN server, a wireless communications system, and a terminal attach method are provided. The RAN server configured to centrally manage base stations of a plurality of wireless communications standards is introduced, and the RAN server is used as a central node to implement a unified interface to a core network. A base station that is of any wireless communications standard and that is managed by the RAN server may independently provide a data transmission service to a terminal, so that the wireless communications system provided in this application basically does not cause an additional latency. In addition, a TAI unrelated to a wireless communications standard of a base station is carried in an attach request message, an attach accept message, and an attach complete message that are exchanged in a terminal attach process.