Abstract: Proper description of welding heat input distribution on the workpiece surface is a prerequisite for precisely analyzing welding thermal process.Based on the arc physics and the interaction of droplets with the weldpool,a bimodal distribution model of GMAW welding heat input at the deformed weldpool surface is established,and the volumetric distribution of droplets heat content inside the weldpool is determined.Then a numerical analysis model of the fluid flow and temperature fields of GMAW weldpool is set up in this paper.Numerical simulation technique is employed to study austenite grain growth process in HAZ of mild steel and HQ130 steel,and to give out the calculation results of austenite grain size in different points of the HAZ under the condition of the different welding heat input.Experimental results indicate that the predicted weld thermal cycle and austenite grain size of the HAZ agree with the measured results.