A power supply system for an electronic device includes a main device and at least one slave device coupled to the main device. The main device includes a power module and a detection module. Each slave device includes a slave control module, a determination module, and a switch module. The power module supplies power for the slave devices via the switch module. The detection module detects a total consumption of power of the coupled slave devices and compares the total of consumed power to a power rating of a power source. Each determination module is configured to determine whether a posterior slave device is coupled to the local slave device. Each slave control module can deactivate the switch module of the local slave device when the total of power consumed exceeds the power rating and the local slave device disconnects from the posterior slave device.