Abstract: The finite element model for the welding process of SS400 medium steel plate was established by using the numerical simulation technique. The body heat source model combined with birth-death model was adopted for the simulation of weld thermal cycle process under different heat-input conditions. According to the simulated characteristic temperature, the change law of heat affected zone(HAZ) width was obtained. The simulated and experimental results were compared. The results show that the influence of heat-input on the width of overheated zone was little; the width of incomplete recrystallization zone increased linearly with the increase of heat-input. The error between the simulated values and test values of thermal circling curves was below 4.5%. The simulated width for the welding HAZ was accorded with test value, which proved the accuracy of modeling and loading.