Abstract: We established a three-dimensional macro heat transfer model through the method of finite difference, and then the information of the temperature field which is needed in the calculation of nucleation and growth of dendrite was obtained by further refining the mesh. The micro model of nucleation and growth was established based on the cellular automaton method. The nugget microstructure simulation was set up by coupling the macro temperature field model with the micro cellular automaton model. It is successful to simulate the formation of resistance spot welding microstructure by using this model, and the simulation results were verified by means of real time image. The numerical simulation results indicated that the nuggets of 5754 aluminum alloy were made up of columnar-equiaxed crystals, which were consistent with the experimental results.