Abstract: The signals of electrode displacement and welding current were sampled by the improved monitoring system of AC resistance spot welding(RSW).The fluctuation characteristics of the electrode displacement were analyzed,and it was concluded that the displacement fluctuation was caused by 50 Hz AC resistance heat pulse.The mechanism of electrode displacement fluctuation was analyzed by onion phenomenon in RSW,and the results showed that the electrode displacement was mainly caused by thermal expansion before the nugget formation and by phase transition expansion after that,and there were the fluctuation characteristics in both expansion.The power factor angle,dynamic resistance and dynamic resistance heat were calculated with the firing angle and conduction angle provided by the welding current curve.Through the comparative analysis on the peak of the displacement fluctuation cycle and the dynamic resistance heat,it was found that the former was sensitive to the thermal and phase transition expansion,and can be used to reflect the different stages of the nugget formation process.