In this study, a coupled axisymmetric finite element model (FEM) was built to sim­ulate the resistance spot welding (RSW) process between ultra-high strength hot-stamped (UHSS) and mild steel by SORPAS software. Via simulating this process, the temperature distribution and dynamic temperature curves of the welding area were studied, and welding spatter phenomena were predicted and validated by com­paring them with experimental results. By adjusting the welding parameters in nu­merical simulation, appropriate welding parameters were achieved. Moreover, the mechanical properties of the welding joints under the optimized conditions were also compared with those of not optimized. The study results have already been applied in a manufacturing production. It can also provide guidance for the RSW on UHSS and mild steel.