Embodiments of the present disclosure provide a data transmission method, user equipment, and an access network device. UE transmits air interface data to an access network device by using a serving cell set including at least two core serving cells that can independently serve the UE to transmit data, and initiates an RRC connection re-establishment process to the access network device only when detecting that all core serving cells in the serving cell set are unavailable. Data transmission is performed by using an available serving cell to the greatest extent. In other words, data transmission can be performed by using any available core serving cell, to reduce the dependence of data transmission between serving cells and minimize possibility of data transmission interruption, thereby improving data transmission robustness.