Abstract: When an arc grounding fault occurs in a flexible grounding system, after the parallel small resistor is put into operation,the system grounding mode is changed from the small-current grounding mode to the large-current grounding mode. The current at the point of fault further increases, and the effect of the input of the parallel small resistor on the fault characteristics of the arc needs to be further clarified. The traditional black box model is difficult to accurately calculate the nonlinear fault characteristics of the arc,that is, it is not easy to obtain the analytical solution of the fault state equation. To solve this problem, the arc black-box model can be transformed into a controlled source equivalent model to establish the arc-grounding fault equivalent circuit of a flexible grounding system. Then the analytical expressions for the zero sequence voltage of the bus and the zero sequence current of each line are given to analyze the arc fault characteristics of the flexible grounding system before and after the shunt resistor is put into operation. It is found that the harmonics introduced by arcing in the zero-sequence voltage are more obvious after the shunt small resistor is put into operation. In contrast, the arcing characteristics of the zero-sequence current of the faulted line are weakened. Finally the simulation verifies the conclusions’ correctness and provides a theoretical basis for the subsequent research on the arc-grounding fault protection of the flexible grounding system.