Abstract: 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 calcutled results agree well with the experimental results.