Abstract: The control mode of AC PMIG welding is a pulse with a droplet in a cycle.Pulse current is designed in arc EP time,while the other time is basic current in a cycle.Dynamic process of current is controlled quickly by the first inverter,and arc polarity is controlled by the second inverter of arc welding power source.Double closed-loop is used in control system.Arc current is controlled by inner loop,and arc voltage and arc polarity is controlled by outer loop through 80C196KC single chip microcomputer.According to the signals of arc voltage errors and arc state,four control regulations and counting measure of control parameters are designed.Varying frequency is used to control AC pulse frequency in normal welding state.It has been proved that the welding process has high stability and even droplet transfer with the proposed control scheme.