Abstract: The interfering mechanism of plasma arc with pilot arc has been studied in Vertical-up syncrowave A. C. PAW in this paper. It shows that the directions of currents and corresponding electrical fields of both plasmd arc and pilot are are opposite as the polarity of workpiece is changed into negative, so that the accumulating charges adjancent to the tip of tungsten and the surface of orifice will be randomly changed, which may be favourtlble forplasma arc but not for pilot arc, or on the contrary, or unfavourable for the both, or favourable for the both. All the Se will make the plasma arc unstable. Although the phenomenon of arc interference is different from that of double arc, the reason which causes the interference is the same as that do es the double arc,i.e.the potential of nozzle is not adaptable to the electrical field of plasma arc. The interference can be efficiently avoided by uncontinuous pilot arc which can make the potential of nozzle adaptable to the plasma arc,decrease the phenomenon of double arc and improve the stability of weld formation in keyholing PAW process.