A rated power supply system powered by PoE receives power from external power sources and supplies power to PDs. The system connects with PDs through PoE output interfaces. The system has a PoE analog controller to turn on/off of the output power of all the PoE output interfaces. The PoE analog controller also detects the output current of all the PoE output interfaces. The system has a packet switch controller, a power state detecting circuit and a voltage conversion circuit. The voltage conversion circuit merges the power received from external power sources to generate a first voltage. The CPU calculates an output current upper limit based on the first voltage. The CPU gets a total output current from the PoE analog controller. If the total output current exceeds the output current upper limit, the CPU will turn off the PoE output interfaces according to power output priorities of the PoE output interfaces.