It is important to measure the capacitance to ground accurately for arc-suppression coil’s automatic control. The principle area of impedance triangle based on impedance angle difference is proposed to calculate the system capacitance to ground aimed at coil series resistance grounding system. Triangle is constructed based on impedance angle difference by changing the value of inductance or resistance of arc-suppression coil, measuring the magnitude and phase of zero-sequence current before and after arc-suppression coil regulation. Unbalanced voltage is solved to calculate ground parameters. Matlab simulation results show that the method is easy to adjust and simple to calculate. This method meets the requirements of online measurement.