Embodiments of the present disclosure provide a communication method, a network side device, and a user terminal. The method includes: establishing, by a user terminal, a connection to a first base station by using a first component carrier; sending, by the first base station, a preprocessing indication message to the user terminal, to indicate that the user terminal is to perform uplink synchronization with a second base station on a second component carrier; and obtaining, by the first base station, a target second base station, and sending a first indication message to the target second base station, to indicate that the user terminal is to perform data transmission with the target second base station.