Abstract: The paper deals with the establishment of mathematical model of spot welding for quality control. The power factor method of quality control for spot welding is studied,and the mathematical model of controlled object is established based on the theory of process discrimination.Transfer function of spot welding has been developed in this paper,which takes SCRs control angel as input qaulity and change rate of dynamic power factor as output quantity.This lays a theoritical foundation for design of spot welding quality contyol system.