Abstract: The hydraulic control rod drive technology (HCRDT) is a newly invented patent of Institute of Nuclear and New Energy Technology of Tsinghua University which possesses independent intellectual property rights. The hydraulic cylinder is the key part of this technology. The step-down motion affects the reset of the hydraulic cylinder so the performance of the hydraulic cylinder directly affects the function of the HCRDT. The theoretic model was obtained and the pressure transient was analyzed. Based on the abnormal conditions occurring in the operation of the mechanism, the working performance was obtained and the results were verified by the experiment. The results show that the displacement of the inner cylinder decreases to stop moving and the speed of inner cylinder decreases when the pressure decreases. The flow velocity of sealing ring decreases because the pressure drops between the two surfaces of the inner cylinder due to the pressure decrease of the inner cylinder. Thus, the model can calculate the physical variables which can analyze the performance of the mechanism.