Abstract: Control rod hydraulic drive system (CRHDS) is the key power control equipment of 200 MW Nuclear Heating Reactor (NHR200), and the capacitance rod position measurement sensor (CRPMS) is the key measurement equipment of CRHDS. For the measurement error caused by the eccentricity of the measuring rod, a new measurement method which can separate the capacitance variation caused by the stepping and eccentricity of the measuring rod was proposed (eccentricitystepping separation method). Based on this method, a design of fourhelix electrodes CRPMS was proposed. For the sensor, the theoretical model was established by finite element method, and the static characteristic experiment was carried out. The calculation results of the theoretical model are in good agreement with the experimental results. Based on the theoretical model, the function between capacitance values and rod positions was established, and the measurement error of the sensor was systematically analyzed. The structure optimization of the sensor was conducted. The results show that for the fourhelix electrodes CRPMS with eccentricitystepping separation method, the maximum measurement error is -3.11 mm under the calculation conditions, which can meet the requirements of rod position measurement without losing step. The results provide guidance for the improvement of CRPMS.