Abstract: During commutation process of variable polarity plasma welding welding test, the phenomenon of welding current resonance are observed and the resonance peaks of current are much different with different polarities, i. e., the welding system has varied damp factor.The phenomena are explained by the arc physics theory and commutation circuit model.The commutation circuit structure is redesigned to adjust the damp factor of system after commutation, while the commutation speed and arc stability are not influenced.By this way, the peak of current is reduced and the working condition of commutation component is improved greatly. The research is instructive for design of high-power variable polarity plasma arc welding power supply.