Keyhole　effect　and　its　periodical　fluctuation　are　important　characteristics　of　deep　penetration　laser　welding　processing.　The　plume　fluctuation　over　laser　keyhole　arises　from　the　pressure　fluctuation　of　keyhole　gas.　It　is　validated　by　plume　behavior　obtained　from　a　series　of　plume　images　by　a　high-speed　videography　system,　and　the　expanding　speed　of　hot　gas　emitted　from　laser　keyhole　in　the　workpiece　is　determined.　Then,　a　mathematical　model　for　the　simulation　of　characteristic　weld　gas　shield　phenomena　during　deep　penetration　laser　welding　based　on　a　numerical　solution　of　the　equations　of　species,　mass,　momentum　and　energy　is　presented.　Important　differences　are　observed　between　Ar　gas　shield　and　He　gas　shield.　All　these　experiments　are　also　simulated　using　the　3D　hydrodynamic　software　Fluent.　It　is　seen　that　the　characteristic　size　of　argon　gas　shield　is　larger　than　that　of　helium　gas　shield,　and　the　smaller　the　nozzle　inclination　angle,　the　larger　characteristic　shield　size　of　the　assist　gas.　The　effective　protection　zone　will　be　increased　when　the　flux　rate　of　the　nozzle　assistant　gas　is　increased.　The　expansion　speed　of　the　plume　over　laser　keyhole　is　much　smaller　than　150　m/s.　The　maximum　mass　fraction　in　the　species　flow　zone　is　smaller　than　0.9.