Based　on　the　theory　of　fluid　dynamics,　thermal　mechanical　coupled　model　of　weld　pool,　keyhole　and　flow　field　of　K-PAW　(Keyhole　Plasma　Arc　Welding)　are　studied　with　FLUENT　software　in　this　paper.　A　optimized　model　of　energy　and　arc　pressure　changed　quadratically　as　the　depth　of　keyhole　increasing　is　proposed.　Furthermore,　the　forces　maintaining　keyhole　stable　act　inside　of　the　keyhole　and　on　the　physical　boundary　at　the　same　time.　The　numerical　simulation　of　penetration　process　and　welding　process　after　penetration　are　realized　initially　with　this　method.　It　takes　2.15　s　to　penetrate　the　whole　workpiece　at　the　calculated　result　which　is　in　agreement　with　the　penetrating　time　of　experimental　measurement.　And　the　forms　of　keyhole　agree　well　during　dynamic　penetrating　process.　The　trend　of　fusion　line　is　same　basically.　©　2017,　Editorial　Board　of　Transactions　of　the　China　Welding　Institution,　Magazine　Agency　Welding.　All　right　reserved.