In　order　to　accurately　simulate　the　process　of　laser　deep　penetration　welding,　a　mathematical　model　to　describe　laser　deep　penetration　welding　was　developed　by　using　the　heat　source　derived　from　the　ray-tracing　model,　and　taking　account　into　the　effect　of　keyhole　on　welding　pool.　With　all　the　governing　equations　including　continuity,　momentum　and　energy　equation,　the　VOF　method　is　adopted　to　trace　the　free　surface　of　the　molten　pool.　Numerical　simulation　was　conducted　by　FLUENT　6.3　software　package.　The　simulation　results　show　that　the　formation　of　keyhole　in　the　weld　is　caused　by　recoil　pressure.