Mathematical　statistics　were　used　to　study　the　stability　of　weld　pool　and　the　elimination　of　weld　defects　in　aluminum　alloy　plasma　arc　keyhole　welding　at　continuously　varying　positions.　In　the　mathematical　model,　the　mass　transfer　position　and　spatial　welding　position　were　taken　as　the　input,　and　the　shape　of　the　welded　joints　(symmetry/deviation)　was　taken　as　the　output.　The　results　showed　that　the　fitted　curves　of　the　front,　back,　and　average　deviations　of　the　weld　seam　were　all　similar　to　the　actual　curves.　According　to　the　optimum　results　obtained　in　the　experiment　and　the　mathematical　models,　the　mass　transfer　position　only　needs　to　be　adjusted　once　(near　to　30　&　DEG;)　during　the　continuously　varying　positions,　from　vertical-up　to　horizontal　welding.　A　breakthrough　from　fixed　environmental　variables　to　dynamic　environmental　variables　in　the　process　control　of　the　keyhole　weld　pool　was　realized,　which　enabled　the　Al-alloy　keyhole　weld　pool　to　resist　the　disturbance　caused　by　gravity　during　variable　position　welding.　The　deviation　of　the　welded　joints　of　the　whole　plate　was　smaller　than　0.5　mm,　and　the　mechanical　properties　of　the　weld　reached　at　least　85%　compared　to　those　of　the　base　material,　thus　meeting　the　requirements　of　Al-alloy　welding.