The　variable　polarity　keyhole　plasma　arc　vertical　welding　process　characters　of　aluminum　alloy　were　analyzed　and　the　method　was　found　that　the　variable　cross-section　aluminum　alloy　was　welded　with　the　variable　polarity　keyhole　plasma　arc　vertical　welding　by　precisely　controlling　welding　parameters.　Welding　current,　the　flux　of　plasma　gas　and　welding　speed　were　defined　as　the　adjusted　parameters.　The　dynamic　balance　of　thermal　and　force　in　keyhole　weld　pool　is　the　key　to　regulate　the　welding　parameters　and　realize　the　automatic　welding　of　variable　cross-section　sample.　The　control　device　of　the　VPPA　welding　system　was　constructed　based　on　80C196KC.　So　it　can　regulate　welding　parameters　in　real-time　and　keep　the　balance　between　thermal　and　force　dynamically　and　realize　the　variable　polarity　keyhole　plasma　arc　vertical　welding　process　of　variable　cross-section　aluminum　alloy.