Embodiments of this application disclose a beam time synchronization method, for a terminal device to report, to an access network device, that time synchronization information meets a preset condition. The method in embodiments of this application includes: The terminal device determines the time synchronization information based on a time synchronization value of a first beam and a time synchronization value of a second beam. The second beam is a candidate beam or a previous serving beam, and the first beam is a current serving beam. The terminal device sends indication information to the access network device if the time synchronization information meets the preset condition, where the indication information indicates that the time synchronization information meets the preset condition.