Abstract: Resistance spot welding (RSW) is a metal forming process with multi physical field coupling and invisible. High current in a short welding duration will make improper matching between electrode force and melting of the nugget. Expulsion is a typical welding defects, it will reduce the nugget strength and the using life of the electrode cap. In this study, a piezoelectric actuator assisted resistance spot welding system was proposed and built, which benefit from short response time and high output of the piezoelectric actuator. The force from piezoelectric actuator was added on the main cylinder force during the welding process. In this way, macro static force and high dynamic programmable force regulation were realized. Experiments showed that expected piezoelectric actuator out-puts were obtained. A series of piezoelectric frequency was added into the electrode force. The oscillation of piezoelectric actuator improved the heat distribution in nugget zone. Expulsion was reduced and the nugget sizes increased significantly under the piezoelectric oscillation.