A location update method, and a heterogeneous network communications system and a device are provided herein. The method includes: an information server receives a location update request message from a multimode terminal; the information server searches the pre-stored location area mapping information for the location area information of the second interface; and the information server sends the location update request message of the second interface to a second Paging Coordinator (PC) corresponding to the second interface, where the request message carries the found location area information of the second interface so that the second PC can update existing location area information of the second interface. The system includes an information server, a first PC and a second PC. Therefore, the interfaces of the multimode terminal do not need to initiate location update frequently, message interactions between the terminal and the network are reduced, and not all interfaces of the multimode terminal need to report location area information, thus saving power.