In smart substation, synchronized sampling is the key issues for the relay protection. Protection device depends on the synchronization of external clock source, when sampling data is transmitted via Ethernet for process level. Sampling synchronization method based on voltage vector (SSMBVV) for busbar differential protection is proposed. When the original synchronization method loses effectiveness and the sampling of merging unit (MU) is out of step, sampling data can be synchronized through the phase difference of voltage vector, which can make busbar differential protection keep on working. The synchronization principle is analyzed, the system wiring diagram is introduced, and the implementation details for SSMBVV are given i.e. calculating the time difference between each MU sampling through the positive sequence voltage; synchronizing the sampling data using quadratic interpolation; synchronization process should be quitted when three-phase PT break is happening. The effectiveness and feasibility of the synchronization method are verified by results of RTDS simulation.