An uninterruptible power supplying method and an uninterruptible power supply. The method includes switching an uninterruptible power supply to a bypass working mode from a main loop working mode, so as to provide mains power for a load through a bypass; determining the working mode of the uninterruptible power supply; and when the uninterruptible power supply is in the bypass working mode, adjusting a working frequency of at least one switch tube in a main loop of the uninterruptible power supply to a second frequency from a first frequency, where the second frequency is less than the first frequency, so as to reduce power loss when the uninterruptible power supply is in the bypass working mode.