Abstract: According to the step motion of single cylinder for Control Rod Hydraulic Drive Mechanism (CRHDM) and the characteristics of the hydraulic cylinder piston ring seal structure, the source of kinetic resistance during step motion process was analyzed, the theoretical model of the dynamic step motion process was built and the value of kinetic resistance based on the results of the control rod hydraulic drive mechanism single cylinder experiment was deduced. By analyzing the relationship between kinetic resistance and the dynamic parameters of the step motion process including the pressure and the velocity of the inner cylinder, calculation model of the kinetic resistance during the step motion process was obtained. The displacement curve inferred from this model agrees with the experimental data. This model would be helpful for the dynamic simulation of the step motion process and the stress analysis of the control rod hydraulic drive mechanism.