The merging unit is a key device employed in digital substation towards voltage-current synchronization. Based on its essential function, this paper gives a new definition of the merging unit, and reviews its relevant standards. Three types of synchronous signal specified are expounded according to actual demands of sampling error of data synchronization. The realization procedure for data synchronization of a merging unit is analyzed based on a modular design conception. The hardware solutions for the merging unit are introduced. Future development trends of the merging unit are briefly described.