Voltage sag has became the most prominent power quality problem, because it may cause great economic loss to enterprises. In this paper, a voltage sag suppression power supply based on multi-winding transformer and power unit was proposed, which could be applied in 10kV dual power supply system. The suppression of voltage sag could be worked by quick switching between the fault and backup AC power sources, and sufficient energy supplying by DC capacitor in power unit during switching time. Then, the parameter design method was discussed, and the correctness and effectiveness of the power supply proposed in this paper was verified by modeling in PSCAD/EMTDC.