The　plasma　arc　evolution　and　the　physical　mechanism　associated　with　variable　polarity　plasma　arc　(VPPA)　remain　unclear,　slowing　down　its　process　development　and　optimization.　In　this　work,　we　revealed　its　increasing　phenomenon　in　the　initial　segment　of　electrode　negative　(EN)　phase　through　the　experimental　measurement　of　VPPA　pressure.　The　current　increasing　enlarged　the　pressure　difference　between　EN　and　electrode　positive　(EP)　phase.　A　unified　numerical　simulation　model　was　developed　coupling　tungsten　electrode,　constricting　nozzle,　plasma　arc　and　workpiece　for　clarifying　the　mechanism　of　pressure　evolution.　In　this　model,　the　electrical　conductivity　of　gas　adjacent　to　melting　region　in　tungsten　electrode　is　considered　to　be　superior　than　that　of　gas　adjacent　to　solid　region.　Through　the　evolution　of　temperature　field　and　energy　transfer,　the　electrode　was　gradually　cooled　mainly　by　thermionic　electron　emission　at　the　initial　segment　of　EN　phase,　because　of　which　the　current　attachment　shrank　to　the　electrode　tip.　Thus,　the　electromagnetic　force　depending　on　the　current　density　increased　in　EN　phase,　leading　to　an　increase　in　the　plasma　arc　pressure.　Our　results　clarified　aspects　of　energy　balance　physics　behind　VPPA,　which　is　critical　for　its　development.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.