Abstract： A kind of software named SORPAS has been applied in this paper to simulate the formation process of nugget in projection welding between hotstamped steel and low-carbon steel. Through the simulation, some changing rules about the temperature field and stress field during the formation process of nugget have been discovered both qualitatively and quantitatively, and the diameter of the final nugget has also been obtained. The results of the simulation show both the shift of nugget to the hotstamping steel sheet of the welding assembly and the change of nugget size with different welding currents applied correspond well with the results obtained by experiments.