The　welding　current,　arc　voltage　and　shape　in　sine　alternative　current　TIG　(SACTIG)　and　variable　polarity　TIG　(VP-TIG)　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.