As the uninterruptible power supply in parallel in the system in all digital control mode, the modulation ratio linear changes in the output voltage is not smooth, at the same time, the voltage detection resolution, the phase synchronization control accuracy and SPWM drive pulse The impact of the dead zone will make the parallel UPS system uninterruptible power supply output voltage amplitude, frequency and phase there is a slight difference. This difference in a single UPS uninterruptible power supply independent operation of its output voltage characteristics of not too much impact, but more than one parallel work, the average load sharing system has a more significant impact, and the corresponding cause of the power module The active, reactive circulation and harmonic circulation. In order to analyze the current sharing characteristics and circulation conditions of the parallel system under the influence of the above factors, the UPS parallel power supply is taken as an example in parallel. The simulation results show that the common load is almost no-load. The main circuit is a half-bridge inverter circuit when the simulation is carried out. The parameter and control system parameters are the filter inductor and the filter capacitor, and the common load is purely resistive. UPS uninterruptible power supply are in open-loop voltage control.