Abstract: In this study, a numerical model for resistance spot welding of DP600 steel was proposed. The thermal and mechanical behaviour at squeezing, welding and holding stages were analyzed. The predicted nugget size was in good agreement with experimental results. The increasing rate of temperature at the center point was the most significant among all locations at the welding stage. Temperature of the center point decreased most slowly at the holding stage. Contact pressure variation at the workpiece/electrode interface and workpiece/workpiece interface during the welding process were very obvious, and could be explained by combining the thermal expansion behaviour of workpieces. A compressive radial stress state was found in weld central region at the end of welding stage, which was due to the thermal expansion mismatch of workpieces. A rapid temperature decrease of the spot weld could be observed at the holding stage. The compressive stress in the central region was therefore partially released. In addition, the workpiece notch and wedged region similar to experimental observation were also successfully simulated.