An uninterruptible power supply supporting active loads includes a charge and discharge module having a battery set and a charger charging the battery set with an input power, a switch circuit having at least two active switches connected to the battery set, a dynamic PWM control module connected with each of the active switches of the switch circuit, alternately outputting duty cycles composed of low-frequency square wave and high-frequency square wave and alternately controlling each of the active switches to turn on or off, and a transformer having a primary side connected with the switch circuit and a secondary side whose two terminals are connected with an output capacitor generating a filtering function in collaboration with an leakage inductor in the secondary side of the transformer. The uninterruptible power supply generates a quasi-continuous output satisfying the hold-up time demanded by an active load.