Abstract: The control drum can be used by the high flux test reactor as the power regulation equipment. The main structure of the control drum is the neutron absorber. For the high flux test reactor designed by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University, by changing the relative angle between the control drum and the reactor core, the reactivity of the reactor can be adjusted. Therefore, the rotation angle of the control drum is one of the most important parameters that should be measured during the operation of the high flux test reactor. The angle signal of the control drum should be identified by the drum position detector. Thus, the control drum position detector is one of the most key safety-related equipment of the high flux test reactor. The capacitive sensing technique is widely used in the industrial measurement, and many physical parameters can be measured, such as the distance, thickness, rod positions, etc. The capacitive sensor has the advantages of the high precision, quick response speed, low cost, etc. Thus, the capacitive sensing technique has a wide application prospect in the field of the nuclear reactor engineering. The capacitive sensing technique can be used to measure the angle signals. In this paper, the primary structure of the capacitive drum position detector was designed based on the arc plates. The detector is composed of the exciting electrode, the detecting electrode, and the measured board. The research methods used in this paper includes the numerical simulation and the theoretical analysis. The numerical model of the detector was established by the COMSOL Multiphysics code. The theoretical model was constructed by the conformal mapping. The error of the drum position detector in the range of 0°-180° was investigated based on the numerical model. The impact scope of the fringe effect of the drum position detector was clarified. The method that can compensate the fringe error was proposed by adding an auxiliary drum position detector. The error characteristics of the auxiliary drum position detector in the range of 0°-180° was analyzed. The structure optimization design of the drum position detector was conducted, and the performance of the capacitive drum position measuring system was obtained. The results show that when the fringe error is compensated, the maximum measuring error of the capacitive drum position measuring system is smaller than ±0.004° in the range of 0°-180°. The measuring accuracy of the position of the control drum in the high flux test reactor can be satisfied.