Abstract: The key-hole in the end of friction stir welding (FSW) joint is one of the application and promotion barriers. For overcoming the barrier, a new technology based on the theory of pressure welding for filling key-hole is invented and researched on the three phase secondary rectifier resistance spot welder in the paper. The key-hole in the friction stir spot welding lap joint of 3.0 mm + 3.0 mm thickness 2024-T4 aluminum alloy is filled. In the paper, micro hardness and micro structure about the filled joint are all studied, and the bonding mechanism is analyzed according the welding theory and crystallography. The results prove that the filled joint including the fusion welding zone, pressure welding zone, plastic deformation zone and melted stopper zone. The diffusion welding is the major metallurgy bond and fusion welding is the minor form of bond between the stopper and key-hole. The micro hardness and structure around the filled key-hole are not changed by the high-efficiency filling technology.