Abstract: A spot welding servo pressure device was designed to solve the fluctuation and non-control of pressure during spot welding process with pneumatic welding gun.The developed device had the advantages of soft contact between the electrodes and workpiece,quickly imposing pressure and accurate pressure control.Dynamic analysis was carried out on the designed servo pressure device,and a transfer function model based on the control mode and equivalent reference model based on the dominant pole were established.The relationship between the mechanical parameters of the servo pressure device and its damping ratio and natural oscillation frequency was acquired,and the influence of mechanical parameters on the system stability and dynamic characteristics was also analyzed.