Variable　polarity　plasma　arc　(VPPA)　is　a　kind　of　source　to　provide　heat　and　force　at　welding　process.　It　can　remove　the　oxide　layer　with　high　melting　point　on　the　surface　of　base　metal　using　the　cleaning　action　of　cathode　spots　(the　special　property　of　VPPA).　So　variable　polarity　plasma　arc　welding　(VPPAW)　is　a　very　suitable　method　to　join　aluminum　alloys　which　always　have　extremely　tenacious　surface　oxides.　It　is　great　significant　to　understand　clearly　the　physical　characteristics　of　VPPA　for　predicting　welding　defects　and　making　the　welding　process　stable.　Therefore,　modeling　and　simulating　VPPA　are　necessary　and　helpful　to　understand　welding　process　theory　and　promote　its　application　further.　In　this　work,　a　three　dimensional　transient　calculated　model　of　VPPA　was　established.　To　describe　the　electrical　characteristics　of　VPPA　at　different　polarities,　a　sequential　electric　conducting　model　was　proposed.　With　finite　difference　method,　the　temperature　field,　fluid　flow　and　current　density　of　VPPA　were　solved　out.　And　the　distribution　of　plasma　arc　pressure　on　the　anode　surface,　as　well　as　its　evolution　process　as　the　time　going　on　were　analyzed.　Arc　pressure　was　measured　experimentally　to　verify　the　calculated　model.　The　results　show　that　the　arc　temperature　field　of　electrode　negative　(EN)　is　more　compressed　than　that　of　electrode　positive　(EP).　The　range　of　high　temperature　at　EN　is　a　little　larger.　Arc　pressure　and　current　density　of　EN　at　central　area　are　both　higher　than　EP.　Nonetheless,　the　magnitude　of　these　values　begins　to　reverse　at　a　certain　distance　to　center　in　radial　direction.　Moreover,　the　arc　pressure　rapidly　responses　to　welding　current.　Pressure　at　EP　is　about　20%　lower　than　that　of　EN.　The　pressure　reduces　to　the　lowest　value　when　the　current　pass　through　0.　After　that,　while　the　current　reaches　to　normal　value,　the　pressure　will　immediately　impact　to　a　larger　value,　then　quickly　recover　to　an　average　value.　Otherwise,　to　compare　the　experimental　results　with　calculated　results　of　arc　images　and　arc　pressure,　they　are　in　good　agreement　with　each　other.