An energy storage system and a power supply method thereof are provided. First and second energy storage devices are connected in series between an AC power source and a load. When the AC power source is normal, the AC power source is used to charge the first energy storage device and provided to the second energy storage device for charging. When the AC power source is abnormal, an output of the first energy storage device is switched from the AC power source to a first energy storage power source, and the second energy storage device is disabled from using the first energy storage power source for charging. A power source provided by the first energy storage device or a second energy storage power source provided by the second energy storage circuit is output to the load according to a power supply state of the first energy storage device.