Abstract: The welding current, arc voltage and shape in sine alternative current TIG (SACTIG) and variable polarity TIG (VPTIG)welding were collected by high-speed camera system and welding arc analyzer.Electrical arc stability model of VPTIG was established based on the variance ratio of average conductance of arc, the relationship between welding arc stability and relevant parameters was compared and analyzed during the welding current to zero of SACTIG and VPTIG.The research showed that the arcs of VPTIG and SACTIG essentially were same as the arc of ACTIG, and therefore both relevant parameters on the stability of arc were consistent; There were differences between arc of VPTIG and SACTIG in many aspects such as heat production, heat output, change of polarity, floating voltage of welding power source, welding current waveform and so on, so their arc stability represented many different characteristics. The arc stability of VPTIG and SACTIG were improved as the rise slope of floating voltage which VPTIG welding power imposed on two electrodes and frequency of floating voltage of SACTIG welding power increased.The higher resignation voltage would cause the more vertiginous arc, the higher floating voltage and welding current would cause the steadier arc.