A power failure prevention system includes a switch circuit, an energy storage circuit, a voltage detector circuit and a control circuit. The switch circuit includes a first switch, a second switch, a third switch and a fourth switch. The energy storage circuit is connected to the switch circuit. The voltage detector circuit detects an input voltage provided by an input voltage source and a stored voltage of the energy storage circuit. The control circuit controls the switch circuit according to the detected input voltage and stored voltage. When the input voltage is higher than a first threshold, the switch circuit allows the input voltage to charge the energy storage circuit. When the input voltage is lower than a second threshold, the switch circuit allows the stored voltage to discharge to the input voltage source. The first threshold is higher than the second threshold.