The present invention discloses an on-line uninterruptible UPS system, comprising an AC-DC conversion circuit, a DC-AC conversion circuit and a backup battery circuit, characterized in that the backup battery circuit comprises a battery group and two switch devices, the two switch devices are connected in series, and then connected in parallel with the battery group, the two switch devices are turned on complementarily to each other, the two ends of the battery group are connected to a positive voltage output end and a negative voltage output end respectively, and a neutral line end is connected in between the two switch devices. In the present invention, since two switch devices are connected in series and then in parallel to a battery group, by complementary turning on of the two switch devices, the one battery group can achieve the function of time-sharing operation of two battery groups, such that the number of the batteries in the UPS system can be reduced by half.