The deficiencies to traditional un-interruptible power supply charging was described. Instead of the rectification and charging as well as PFC three circuit modules in traditional ups,the PWM rectifier was used and by controlling it we achieved three-stage charging.Meanwhile,in order to realize smooth switching in each charging stage,the current closed-loop sliding-mode control strategy based on synchronous rotating coordinate was adopted.simulation result shows that this control strategy is of better DC charging voltage tracking capability with smooth response and strong robustness.