Abstract: In this paper, the arc stability of AC composite plasma arc welding by a single AC source and the arc stability of AC plasma arc welding with general pilot DC source were compared. Bascd on this comparison, the authors further studied the mechanism of arc stability of AC composite plasma arc welding. The results show the arc stability of AC composite plasma arc is stabilited by the combined effect of the "heat" produced by the AC pilot arc and the electropotence produced in the reactor.