UPS power system consists of 4 parts: rectification, energy storage, transformation and on-off control. UPS realizes the function of voltage stabilization with rectifier, which is generally a silicon-controlled rectifier (SCR) or high-frequency switching rectifier. The rectifier controls the output amplitude according to the change of external power supply; when the external power supply changes (the change should meet the system requirements), it outputs a rectified voltage whose amplitude keeps unchanged. Purification function of UPS is completed by the energy storage battery. As the rectifier cannot eliminate instantaneous pulse interference, the rectified voltage still has interference pulse. Energy storage battery can not only store DC power, but also plays a role of large-capacity capacitor for the rectifier, its equivalent capacitance is proportional to the capacity of energy storage battery. As the voltage at both ends of the capacitor can’t change suddenly, the pulse interference is eliminated due to the smoothness property of capacitor for pulses, thus realizing the purification, as well as the shielding against interference. Converter guarantees a stable frequency of uninterruptible power supply. Frequency stability is determined by the stability of the converter's oscillation frequency. To facilitate daily operation and maintenance of UPS power system, an operating switch, automatic bypass switch after fault self-checking, overhaul bypass switch are designed in this system.