Abstract: Based on theories of hydromechanics and heat transfer,a three dimensional transient numerical model of flow and temperature field of the keyhole plasma arc welding (PAW) molten pool with a moving arc is established,and the model is discretized with the finite element method.In the model,both convection heat transfer of the melted metal inside the pool and heat conduction of solid metal outside the pool,the variation of thermophysical parameters of material with temperature,the effect of the loss of heat,including convection and radiation,and latent heat of phase fransformation on the temperature field are considered.Welding heat source is presumed to be gaussian distribution.Also,the boundary conditions are dealed in the calculation. On the bas is of the model,elements are divided according to a non-uniform grid and time steps are defined to a non-uniform step in different subareas.ANSYS,calculating software of finite element technology,is adopted.The results show that the calculated results approximately agree with the experimentally measured geometric sizes and shape of the fusion boundaries,and that the model is basically correct and credible.