The　variable　polarity　plasma　arc　(VPPA)　welding　process　is　well　suited　for　the　welding　of　aluminum　alloy,　but　the　stable　welding　process　and　good-quality　weld　bead　are　difficult　to　be　obtained　in　the　flat　position.　The　purpose　of　this　work　is　to　study　the　characteristic　of　variable　polarity　plasma　arc　pressure　with　different　polarity　time　and　find　the　key　fact　to　achieve　welding　process　stability　and　good　quality　welding　joint　in　the　flat　position.　The　arc　shape　variation,　arc　pressure　evolution　and　the　molten　pool　behavior　were　observed　and　measured　to　discover　the　relationship　to　the　weld　joint　formation.　It　was　found　that　the　small　plasma　arc　pressure　in　the　center　with　a　similar　affected　region　and　enough　upward　shear　force　of　arc　plasma　are　the　necessary　conditions　to　get　a　stable　keyhole　process,　which　could　drive　the　liquid　metal　of　the　back　side　weld　pool　converges　at　the　bottom　of　the　keyhole　and　flows　upward　and　form　a　high-quality　weld　bead.　Comparing　the　welding　stability　and　quality,　adjusting　polarity　time　is　the　good　method　for　completing　the　necessary　conditions.　With　the　variable　polarity　time　decreasing,　the　stable　keyholes　process　and　acceptable　weld　quality　were　obtained　in　the　flat　position.