A power supply for a negative voltage load has a switch-mode power unit, a monitoring unit, a first auxiliary power unit and a second auxiliary power unit. The switch-mode power unit has a rectifying circuit, a power factor correction circuit, a DC to DC power circuit. The first auxiliary power unit is connected to output terminals of the power factor correction circuit and converts DC power to a first DC power for driving the monitoring unit. The second auxiliary power unit is connected to the negative voltage load in parallel and converts negative power from the negative voltage load to a second DC power for driving the monitoring unit when AC power is interrupted. Since the first and the second auxiliary power unit do not work at the same time, the power supply can effectively enhances the conversion efficiency of the power supply.