Abstract: The dynamic resistance signal was collected by an automatic data acquisition system.Research indicated that the dynamic resistance signal responded well to the change of welding parameters, and contained plenty of quality information.There was an obvious correspondence between the dynamic resistance signal and the growth process of nugget.The inflexion time and destination value of dynamic resistance curve had strong relativity with the time of nugget appearance and the final size of the nugget respectively. Welding parameters affected the dynamic resistance signal obviously and regularly.When an expulsion occurred, a sharp drop in dynamic resistance curve and a peak in the change rate of dynamic resistance curve can be used to automatically identify the splash.