12 groups of welding process parameters (welding current at 60, 70, 80, 90, 100, 110 A, welding speed at 1.96, 2.88 mm·s-1) were set by uniform experimental design. The welding finite element model of DP780 dual phase steel was established. By the simulation and experiments, the optimal welding current and welding speed were determined with the minimum deformation of the joint as the index. The temperature field and the content of martensite in coarse-grained zone of heat affected zone (CGHAZ) of the joint were simulated by the welding finite element model, and verified by the thermal simulation experiment. The results show that the deformation of the welded joint was the smallest with the welding current of 100 A and the welding speed of 1.96 mm·s-1, and the relative error between the finite element simulation and the experimental result was 3.528%, indicating that the model was accurate. By the finite element simulation, the area with a distance of 2.44-6.97 mm from the weld center was the CGHAZ. The area fraction of martensite in CGHAZ was 43%, which was close to that (44%) in the CGHAZ sample prepared by the thermal simulation experiment, indicating that the established model could be used to simulate the CGHAZ structure.