An electronic device power supply system includes a plurality of electronic devices and an external power supply. The plurality of electronic devices includes a master device and a plurality of slave devices coupleable to the master device one by one. Each electronic device has a power node. The master device includes a power module capable of supplying power for one of the slave devices coupled to the master device and next slave device via the power point. When each slave device is coupled, the slave device sends local rated power to the master device. The power module stops suppling power to a last coupled slave device when the rated power of the last coupled slave devices is over the surplus power of the master device, and the external power supply is coupled to the last coupled slave device.