The dynamic mechanical responses of resistance welding machine,which is mainly governed by the mechanical parameters of the machine, is very important to the weld quality especially in projection welding when collapse or deformation of work piece occurs. In this paper, a mathematical model for characterizing the dynamic mechanical response of resistance welding machine and a special test set-up called breaking test set-up have been developed. Based on the model and the test results, the mechanical parameters of the machine were identified, including the equivalent mass, the damping coefficient, and the stiffness for both electrode systems.