Abstract: The paper introduced circuit structure,working principle and control strategy of a new type of single-phase UPS system with series-parallel dual-inverter. The two inverters of the UPS system could work in coordination with each other in inverted mode when power net was cut, and the parallel inverter runs in on off state of 50 Hz pulse wave, its harmonic was compensated by the series inverter which runs in high frequency PWM state. The output wave of the UPS system was still in sine wave,and the efficency of the UPS system was improved. The simulation research proved the feasibility of this scheme.