A control method of an uninterruptible power supply (UPS) for extending a discharge time under a no-load condition instructs a battery of the UPS to discharge by repeating an alternate on and off cycle when the UPS enters a battery mode and after a no-load condition continues over a period of time so that the UPS alternately and repeatedly discharges and stops discharging based on the alternate on and off cycle, and detects if the no-load condition is not present during the discharging period. If not present, the battery is restored to continuously discharge. Additionally, the discharge voltage can be reduced during the discharging period to alleviate the power discharged under the no-load condition. Accordingly, the control method can reduce to discharge under the no-load condition and further extend a discharge time under a no-load condition.