The invention relates to an online uninterrupted power supply comprising an alternating current input coupled with a mains input end, an alternating current output coupled with a load end, a rectifierThe invention relates to an online uninterrupted power supply comprising an alternating current input coupled with a mains input end, an alternating current output coupled with a load end, a rectifierdevice used for the rectified output of the inputted alternating voltage, a direct current/direct current conversion device, a direct current device used for providing direct current voltage when mai device used for the rectified output of the inputted alternating voltage, a direct current/direct current conversion device, a direct current device used for providing direct current voltage when mains power is off and a first switch device which is powered by a direct current power supply when being conducted, a direct current/alternating current conversion device and a second switch device usedns power is off and a first switch device which is powered by a direct current power supply when being conducted, a direct current/alternating current conversion device and a second switch device usedfor switching the power supply between the mains power and the uninterrupted power supply; wherein the direct current/direct current conversion device is used for converting input direct current volt for switching the power supply between the mains power and the uninterrupted power supply; wherein the direct current/direct current conversion device is used for converting input direct current voltage into sine-wave pulse width direct current voltage which is processed into sine-wave pulse width alternating current voltage by the direct current/alternating current conversion device and then outage into sine-wave pulse width direct current voltage which is processed into sine-wave pulse width alternating current voltage by the direct current/alternating current conversion device and then output after being filtered, thus reducing the power conversion loss; and the direct current/direct current conversion device also can be used for conveying excess energy generated by a load side or theput after being filtered, thus reducing the power conversion loss; and the direct current/direct current conversion device also can be used for conveying excess energy generated by a load side or thedirect current/alternating current conversion device back to the direct current power supply for storing energy, thus realizing power-saving effect.direct current/alternating current conversion device back to the direct current power supply for storing energy, thus realizing power-saving effect.