Abstract: The differences between each super capacitor can result in over-voltage break down of individual capacitor, and then makes whole capacitor series invalid soon. Commonly used equalizing circuits often work in linear mode, and the circuit is complex, and not easy to debug and manufacture. A practical and high efficient circuit is designed for balancing the voltage on each capacitor in a series. This circuit works in switch mode and consists of only 5 components. So, it has been applied in the power supply circuits of sonar transmitter to protect capacitor series for a long time.