Using　YB005-01　pressure　transmitter　the　plasma　arc　force　in　positive　polarity　was　greater　than　that　in　reverse　polarity　was　measured.　The　thermal　and　force　models　on　aluminum　alloy　variable　polarity　plasma　arc　(VPPA)　keyhole　molten　pool　were　built.　The　stability　of　keyhole　molten　pool　and　the　forming　mechanism　of　weld　under　the　action　of　thermal　and　force　in　asymmetric　positive　and　reverse　polarity　plasma　arc　were　analyzed.　By　further　analysis　of　aluminum　alloy　variable　polarity　plasma　arc　mechanical　and　thermal　characteristics,　the　key　welding　influence　factors　on　welding　current　and　plasma　air　flow　were　mastered.　According　to　the　keyhole　welding　process　testing,　the　reasonably　selected　positive　and　reverse　polarity　current　amplitude　and　plasma　air　flow　which　keep　thermal　and　force　balance　during　keyhole　welding　process　provided　theoretical　support　for　getting　stable　technique　parameters　in　aluminum　alloy　variable　polarity　plasma　arc　welding.