The resistance spot welding process of mild steel was studied by experimental and finite element methods. The results indicated that the relationship between process parameters (such as welding current, electrode force and plate thickness) and nugget size, from which the relationship between process parameters and the quality of joints has been derived. The results also showed the reason for expulsion through theoretical and FEM(finite element method) analysis by taking account of the effect of the contact state on thermal and electrode process. Electromagnetic thermal and structural fields have been carried out to verify the models established for analyzing spot welding, and the results showed that numerical simulation of spot welding is practicable.