Energy storage system and power supply method thereof

A energy storage system includes a power supply device, a main battery and a backup battery, and a power supply method of the energy storage system includes: when the power supply device works normally, a load is powered and the backup battery is floatingly charged by the power supply device, and the main battery is in standby mode; when the power supply device stops supplying power, the load will be powered by the backup battery, and meanwhile, the main battery will be triggered to switch from the standby mode to a backup mode to start supplying power to the load; when an output power of the main battery is equal to a power of the load, an output power of the backup battery is zero and the backup battery is in a bypass state.