In　order　to　accurately　simulate　the　laser　deep　penetration　welding　process,　a　three-phase　unification　mathematical　model　of　vapor　phase,　liquid　phase　and　solid　phase　was　established.　The　VOF(volume-of-fluid)　method　was　employed　to　track　free　surfaces.　Roles　of　surface　tension,　recoil　pressure　and　thermal　buoyancy　were　considered　in　the　model.　Dynamic　process　of　keyhole　and　its　corresponding　temperature　distributions　were　obtained　by　numerical　modeling.　The　welding　experiments　on　304　stainless　steel　alloy　show　the　calculated　results　agree　well　with　the　experimental　results.