User equipment, a base station, a base station access method, and a radio link monitoring method are provided, where the user equipment includes: a first receiving module, configured to receive first indication information sent by a first base station, where the first indication information includes a time period required by the user equipment to initially request to access a second base station; a first sending module, configured to send the random access scrambling code to the second base station; and a first notification module, configured to: if the user equipment fails to access the second base station within the time period, notify the first base station that a secondary cell group failure occurs. In this application, efficiency of accessing the second base station by the user equipment is improved by limiting a time in which the user equipment requests to access the second base station.