The　keyhole　plasma　arc　welding　(K-PAW)　is　widely　applied　in　engineering　project　now　as　a　high　energy　beam　welding　with　its　advantages　of　low-cost　and　easy　operation.　However,　the　arc　instability　may　arise　and　welding　defects　will　be　produced　in　K-PAW　due　to　the　high　current　and　strong　plasma　penetrating　force　when　medium　thickness　plates　are　welded,　finally　weakening　the　efficiency　of　K-PAW.　Furthermore,　it　is　found　that　the　flow　field　of　liquid　metal　in　the　molten　pool　and　the　stability　of　keyhole　have　a　critical　influence　on　welding　quality.　Therefore,　modeling　and　simulating　molten　pool,　keyhole　and　flow　field　in　the　K-PAW　quasi　steady　process　except　for　arc　starting　and　ending　phases　are　helpful　to　understand　the　welding　process　theory　completely　and　promote　its　application　further.　But　to　date,　there　is　little　study　on　the　coupled　analysis　of　molten　pool　and　keyhole　in　the　quasi　steady　welding　process　due　to　the　difficulty　to　make　keyhole　stable.　In　this　work,　based　on　the　principles　of　fluid　dynamics　with　considering　arc　pressure,　surface　tension,　electromagnetic　force,　buoyancy　and　gravity,　a　three　dimensional　transient　model　is　established　to　reveal　the　secondary　changing　of　heat　and　force　effect　regularly　as　the　keyhole　depth　increases.　To　describe　the　welding　heat　process,　a　combined　type　volumetric　heat　source　model　of　＇double　ellipsoid+conical　body＇　is　employed.　A　keyhole　inside　solid　agitated　(KISA)　calculated　method　is　proposed　to　maintain　the　keyhole　stability　in　the　quasi　steady　welding　process.　To　improve　the　computational　efficiency,　the　calculated　region　is　limited　within　the　action　region　of　a　cone-symmetrical　weld　heat　source.　With　volume　of　fluid　(VOF)　method　to　track　the　keyhole　boundary,　the　dynamic　behavior　process　of　molten　pool,　keyhole　and　flow　field　are　calculated　using　FLUENT　software.　The　stability　of　K-PAW　is　analyzed　and　the　factors　affecting　keyhole　production　are　discussed.　The　calculated　results　show　that　under　the　welding　current　140　A　and　plasma　gas　flow　3.5　L/min,　it　needs　3.0　s　to　reach　the　quasi-steady　state　in　which　the　average　thickness　of　molten　pool　in　keyhole　front　wall　is　0.6　mm.　The　offset　range　of　the　keyhole　center　between　top　side　and　bottom　side　is　0.46　similar　to　0.97　mm.　There　is　the　anticlockwise　heat　vortex　appearing　in　molten　pool　of　back　side.　The　calculated　width　of　keyholes　on　the　bottom　side　is　in　good　agreement　with　experimental　results.