Abstract: The technology of filling keyhole based on resistance welding (FKBRW) is a new filling technology for friction stir welding (FSW) keyhole, which can not only achieve the purpose of removing the keyhole but also has a positive effect on the load bearing capacity of the FSW joint. For studying the mechanism of FKBRW, the keyholes of 2024-T4 aluminum alloy friction stir spot welding of 1.5 mm + 1.5 mm were filled on a three-phase secondary rectification resistance spot welding machine for test equipment, and a device monitoring the filling force dynamic behavior was built up. The results showed that there exits two boding forms between the plug and wall of keyhole. The melting connection zone is in the middle of the joints, and the solid phase connection zone is on the both upper and bottom of the joints.The FKBRW technology is a kind of composite welding process that the filling force from upper electrode presses the plug into the keyhole, and the plug goes through the process of softening and pushing and metallurgical combines fully with the wall of keyhole in the last time.