Abstract: The classical theory of welding thermal process was not concernd with the influence of the heat thansferred into the weld pool by filler metal droplets on fluid-and heat-flow in pools.In the theory,the only difference between MIG and TIG thermal processes is embodied in the selection of welding arc power efficiency.In this paper,a new method has been put forward for the first time to account for heat transfer in MIG spot welding,i.e.,the overheated filler metal droplets deliver their excess heat to the weld pool,besides the same spatial instributions of the heat flux on the free surface as that in TIG welding.It also takes into consideration the effect of thermal convection and fluid flow on thermal proceas.The finger penetration phenomenon in the spray transfer of MIG welding has been numerically simulated with the model given in this paper for the first time from the viewpoint of heat transfer and fluid dynamics.