Under the condition of refrequency, the resonant circuit of the capacitor charging power supply oscillates abnormally due to the residual voltage, which leads to switching overcurrent and power failure. To address this issue, on the basis of analyzing the working principle of resonant circuit, we put forward a method of releasing the residual voltage of the resonant capacitor by controlling part of the switching of the power supply itself at the end of each charging cycle. The method not only keeps the initial voltage of the resonant capacitor at zero in each period, but also avoids the defect of adding the discharge circuit. The control method is simple and universal. The circuit simulation and experiment carried out on a 800 V/6 A charging power supply show that the proposed method reduces the residual voltage of the resonant capacitor to zero rapidly after the charging cycle, and the resonant current tends to be stable, which effectively inhibits the abnormal oscillation of the resonant circuit, thus verifying the effectiveness and practicability of the proposed method.