Abstract: The thermal expansion works in the whole welding process.Taking into account the phase transition on the thermal expansion coefficient,a two-dimensional axisymmetric finite element numerical analysis model of the resistance spot welding considering the effects of phase transition was performed.Then based on the thermal results considering the impact of latent heat,the resistance spot welding stress field and deformation of the weldment of dual-phase steel DP600 were analyzed with the thermal elastic-plastic theory and the linear rule of mixtures taking into account of the phase transition.The results show that the analysis result is closer to the actual situation by considering the phase transition on the thermal expansion coefficient.