Abstract: Control rod hydraulic drive system (CRHDS) is a new type of built-in control rod drive technology which is invented by Tsinghua University for NHR200. The integrated valve (Ⅳ) is the main flow control component of the CRHDS. The flow resistance of Ⅳ has a great influence on the control rod dynamic stepup process. The stepup performance experiments of CRHDS with different flow resistances of Ⅳ were conducted under room temperature conditions in the paper. Meanwhile, the theoretical model of hydraulic cylinder stepup process was established to get the dynamic response of the hydraulic cylinder. The calculation results of theoretical model agree well with the experimental data. On this basis, the theoretical model of hydraulic cylinder stepup process was applied to the high temperature working conditions with different flow resistances of Ⅳ. The analysis results show that with the increase of flow resistance, the stepup average velocity gradually decreases, and the stepup time, the pressure lag time after step-up and total time of step-up process increase, which lay a foundation for the design and optimization of the flow resistance of the integrated valve of the CRHDS.