A redundant power supply system has power supplies and holdup control circuits. The power supplies are connected in parallel for connecting to an input capacitor of a load system. An OR-gate anti-reverse current element is connected between each of the power supplies and the input capacitor. The holdup control circuits correspondingly connect to the DC output sides of the power supplies. Each of the holdup control circuits has an inductor, an electronic switch and a controller. The inductor is connected in series to the OR-gate anti-reverse current element. The electronic switch is connected between the DC output side of the corresponding power supply and a node where the inductor and the OR-gate anti-reverse current element are connected. The controller turns on and off of the electronic switch according to the DC power output from the DC output sides, thereby prolonging the holdup time after a power failure.