Abstract: The tuning control of the arc suppression coil depends on the detection of grounding capacitance current in resonant grounding system. Taking a substation in Wuhu as example, the detection method of grounding capacitance current is studied in this paper. The principle and two main methods of tuning are introduced firstly. By considering the influence of accuracy of PT and measurement part when the neutral point voltage is low, an improved tuning method by cutting damping resistance is presented to improve detection accuracy of capacitance current. The feasibility of this method is verified by analysis of equivalent circuit of series resonance with damping resistance theoretically, the detail step is also presented. Finally this method is simulated by MATLAB, and results show that the proposed method is rational and effective.