A redundant power supply circuit includes a port detection circuit, a fixed state detection circuit, a voltage conversion circuit, and a control circuit. The port detection circuit detects and outputs a plurality of ready signals according to a plurality of power signals of a number of POE ports. The fixed state detection circuit outputs first control signals according to a predetermined voltage signal of a network device. The control circuit generates and sends enable signals to the voltage conversion circuit according to the first control signals and the plurality of ready signals. The voltage conversion circuit converts the plurality of power signals into driving voltage signals according to the enable signals. A POE system and method are also provided.