Embodiments of the present invention provides a method for falling back to a 2G/3G network, a relevant device and a system. The embodiments of the present invention includes: receiving, by a mobility management entity, a first message from an access network device, where the first message carries location area information of a target 2G/3G cell to which a user equipment UE prepares to connect; sending a second message for requesting to perform a location update on the UE to a mobile switching center MSC corresponding to a location area of the target 2G/3G cell; receiving a third message from the MSC, where the third message carries a temporary mobile station identity TMSI allocated to the UE by the MSC; and sending, to the UE, a fourth message carrying the TMSI, so that the UE, after being connected to the target 2G/3G cell, communicates with the MSC by using the TMSI.