In a resonant grounding system, calculation of the residual current of the grounding point has guiding significance for prevention of fault risk, the compensation effect evaluation of the arc suppression coil and other maintenance. However, a reasonable technical direction has not been found. Considering the single-phase grounding fault of the resonant grounding system, this paper analyzes the zero-sequence measured admittance at line outlets in the two different stages (normal state of signal injection and arc-ignition state of grounding fault). The analysis shows that the zero-sequence measured admittance at all line outlets when there is signal injection reflects the admittance of the line admittance. Those of the faulty line outlet when grounding fault occurrence reflect the back admittance. Then according to the two admittances and the bus zero-sequence voltage, the active and reactive components of residual current can be calculated. A method for calculating residual current is proposed. Matlab/Simulink simulation shows that the method is basically not affected by detuning degree and transition resistance, and has higher calculation accuracy.