A power supply system and a power supply method thereof are disclosed. The power supply system includes a master power supply device and a slave power supply device. When a master power detection module detects that a current value of a master power signal generated from a master power supply module increases to a first loading proportion, a master micro-controlling module will notices a slave micro-controlling module via a controlling pin to enable a slave power supply module to generate a slave power signal. When a slave power detection module detects that a current value of the slave power signal decreases to a second loading proportion, the slave micro-control module will control the slave power supply module to stop generating the power signal.