Abstract: Hydraulic cylinder (HC) is the key component of the control rod hydraulic drive mechanism (CRHDM). According to the characteristics of HC stepdown process, the pressure transients in the HC during stepdown process can be divided into two processes, that is, the pressure transients before stepdown motion and the pressure transients during stepdown motion. Theoretical dynamic pressure models of these two processes were built. And among them, the theoretical dynamic pressure model during HC stepdown motion can be combined with the HC stepdown kinematics model to establish a complete HC stepdown dynamic model. The comparison between the theoretical results and the CRHDM single cylinder experiment shows that, under the experimental loadings, the theoretical dynamic pressure transients during the HC stepdown process and the HC stepdown displacements agree with the experimental results. The research results have laid the theoretical foundation for the acquisition of the key factors affecting the pressure transients of the HC during stepdown process and the step motion time of the CRHDM.