Welding　defects　easily　appear　in　variable　polarity　plasma　arc　(VPPA)　horizontal　welding　of　aluminum　alloy.　The　influence　of　gravity　on　the　molten　metal　flow　plays　a　critical　role.　Thus,　the　effect　of　the　dip　angle　of　the　torch　and　the　position　of　the　feed　wire　were　investigated.　The　stable　keyhole　of　the　dip　angle　torch　along　the　welding　direction　was　easily　achieved　compared　to　that　of　the　right-angle　torch.　The　distribution　of　thermal-force　input　to　the　keyhole　was　changed　and　the　effect　of　gravity　on　the　molten　metal　flow　decreased.　The　melting　metal　bridging　between　the　upper　and　the　lower　side　became　uniform　by　adjusting　the　position　of　the　feed　wire,　the　metal　liquid　state　was　then　adjusted　to　ensure　that　gravity　is　a　beneficial　force　on　the　molten　metal　flow.　By　introducing　these　optimizing　methods,　stable　keyholes　process　and　acceptable　weld　quality　were　obtained　in　VPPA　horizontal　welding.