According to the present disclosure, a mobility management method, an access network device, and a terminal device are disclosed. A base station system includes a centralized unit and a distributed unit, the centralized unit communicates with the distributed unit, and the distributed unit communicates with a terminal device through an air interface. An example method includes: selecting, by the centralized unit, a target cell as a secondary cell based on a measurement report reported by the terminal device, and sending configuration information of the target cell to the terminal device through the distributed unit; and receiving, by the centralized unit, primary cell change indication information sent by the distributed unit, performing air interface resource reconfiguration for the target cell based on the primary cell change indication information, generating reconfiguration information, and sending, by the centralized unit, the reconfiguration information to the terminal device through the distributed unit.