The　VPPA-1　welding　power　with　80C196KC　MCU　as　the　control　center　and　double　invert　topological　structure　as　the　main　circuit　was　prepared.　Arc　stability　mechanism　and　control　methods　of　VPPA　were　studied.　The　re-ignition　principle　after　the　zero　passage　at　low　current　and　the　arc　stability　of　VPPA　in　large-scale　welding　conditions　were　analyzed　principally.　It　is　proposed　that　zero　passage　should　be　enhanced　before　the　positive　electrode　becomes　negative　electrode　by　program　enactment　at　low　current　to　control　the　stability.　However,　in　the　large-scale　welding　conditions,　the　arc　stability　is　controlled　by　zero　passage.　It　is　helpful　for　the　arc　properties　and　welding　process　control　in　VPPA　welding.