For efficient redundancy power supplying, a voltage drop device, a power selection device, a voltage detection device and a switch are provided in a power control circuit of a redundancy power supply system. The voltage drop device performs a voltage drop operation to convert a redundancy operating voltage into a standby voltage. The power selection device receives the main operating voltage and the standby voltage, and selectively outputting one of the main operating voltage and the standby voltage, which is of a higher level. The voltage detection device outputs a control signal according to a comparison result of the main operating voltage and a threshold. The switch is selectively conducted or interrupted according to the control signal. A voltage drop of power passing through the conducted switch is less than a voltage drop of power passing through the voltage drop device.