An uninterruptible electric power system comprises a main power supply unit, an uninterruptible power supply unit receiving AC commercial power from the main power supply unit, and a slave power supply unit. The uninterruptible power supply unit converts the AC commercial power to stable AC power. The slave power supply unit is connected to the uninterruptible power supply unit. The slave power supply unit comprises a slave static transfer switch, which comprises a first input terminal connected to the uninterruptible power supply unit to receive the stable AC power, a second input terminal receiving the AC commercial power, and an output terminal configured to connect, when the uninterruptible power supply unit is in normal operation, to the first input terminal to receive the stable AC power, and connect, when the uninterruptible power supply unit operates abnormally, to the second input terminal to receive the AC commercial power.