The 3 D finite element model of keyhole plasma arc welding of 316 stainless steel was established by GAMBIT software and meshed,and then the iterative calculation was carried out by FLUENT software. By programming UDF program,the loading of moving heat source and force source and the definition of phase were realized. The VOF equation was used to trace the phase interface. The temperature field nephogram of longitudinal cross-section and transverse cross-section at different time and the flow of liquid metal in molten pool were solved.The results show that:with the change of welding time,the workpiece is gradually penetrated under the action of arc heat,and the liquid metal in the molten pool changes dynamically under the action of plasma arc pressure, plasma flow force and its reaction force. The welding experiment of 6 mm 316 stainless steel was carried out,and the numerical simulation results of the weld cross-section were compared with the experimental results,which basically agreed with each other,which verified the reliability of the numerical simulation results.