An uninterrupted power supply system including a square wave oscillator, a square wave drive, a negative wave drive, a first bridge drive, a second bridge drive, a bridge circuit, a R/C low pass filter, an overload protection circuit, a high-voltage generator, and a full-wave rectifier, wherein the bridge circuit consists of pairs of oxide metal field effect transistors (insulated-gate semiconductors) respectively connected by bridging for the conversion of DC power supply into AC power supply; the high-voltage generator and the bridge circuit are mounted on an expansion card so that the output power of the system can be expanded by installing additional expansion cards.