Abstract: Hydraulic cylinder is the main component of the control rod hydraulic drive mechanism (CRHDM). Step-up and step-down motions are the functions of hydraulic cylinder. The kinetic resistance of hydraulic cylinder during step-down motion is the key parameter in the hydraulic cylinder structure design and step-down analysis. Theoretical analysis of hydraulic cylinder step-down process was carried out to establish the dynamic model of the process. The step-down velocity and acceleration were obtained by combining the step-down dynamic model and the results of the CRHDM single cylinder experiment, which lead to two sets of step-down kinetic resistance models. Comparison of these two models and their calculation results shows that, under the experimental conditions, the displacement curves inferred from these models agree with the experimental data. And in the model based on step-down acceleration the kinetic resistance is the function of inner cylinder’s velocity and the hydraulic pressure inside the cylinder, the relationship of the coefficients in the model to the loadings is easy to obtain, so the kinetic resistance model based on step-down acceleration is more applicable to engineering use and extension.