This disclosure discloses methods, apparatuses, and systems for performing handover. One method comprises: receiving, by a target access network device after a mapping relationship between a quality of service (QoS) flow of a terminal device and a data radio bearer (DRB) changes, a handover request from a source access network device, wherein the handover request comprises indication information for requesting to establish a tunnel; and sending, by the target access network device to the source access network device, information about the tunnel for the target access network device to receive one or more data units from the source access network device after the mapping relationship between the QoS flow and the DRB changes.