The scheme of configuring the Petersen coil after actively measuring the capacitive current of a network has been largely adopted, but the feedback of the compensation state is not usually used. This paper proposes a new method of adjusting the Petersen coil by using the information obtained after arc extinction. Using the equivalent circuit of a single-phase grounding fault in the resonant grounding system, the electrical character are analyzed. It is found that the transient process after arc extinction is the underdamped resonance between the Petersen coil and the capacitance to ground, and the resonant frequency is different from the power frequency. Therefore, the resonant angular frequency and damping ratio of the process after arc extinction can be used to calculate detuning, and the three-phase undervoltage can be used to preliminarily judge whether the system is undercompensated or overcompensated. A method of adjusting Petersen coil to achieve reasonable detuning is given. The effectiveness and accuracy of the theory and the method are supported by simulation and field fault data. The method does not need to add any special equipment or adjust the running state of the existing equipment, and it has high safety.