Embodiments of this application provide a time synchronization method, a service board, and a network device. After clock board times between service boards are synchronized, the service board obtains a CPU time and the clock board time of the service board according to a preset periodicity, and makes a record; when performing CPU time synchronization between the service boards, the service board obtains a current CPU time of the service board, and calculates a synchronization time of the CPU based on the current CPU time, and a last recorded CPU time and clock board time; and adjusts the CPU time of the service board to the synchronization time. After the clock board times between the service boards that need to implement CPU high-precision time synchronization are synchronized by using a high-precision time synchronization protocol, the CPU time of each service board is adjusted to a corresponding synchronization time.