Abstract: A single pass welding process on low carbon steel SS400 was simulated by 3D finite element method using ANSYS. Combined with the SH-CCT diagram, the width, microstructure and hardness of heat affected zone (HAZ) were predicted by the temperature field attained by numerical simulation. The welding experiment was implemented to validate the simulation results. The calculated results show that the width of fusion zone is 0.7~1.6 mm, and the width of coarse grain zone is 1.3~2 mm, and the width of the complete recrystallization zone and partial recrystallization zone are approximately 1 mm respectively. Hence the whole width of HAZ is about 3~4 mm. The prediction of the hardness on coarse grain zone is 180~195 HV. The prediction on microstructure is composed of ferrite, bainite and pearlite. All the prediction results are in good agreement with verification experiments. Consequently, the calculated model and prediction method are reliable for the prediction of HAZ microstructure and its property.