In the face of power quality problems caused by voltage sag, this paper proposes an improved disturbance power method combined with the common type of fault in power system. It redefines the disturbance power by taking the disturbance voltage multiplied with the disturbance current, according to the superposition principle of linear circuit, fault network can be equivalent to the normal plus the disturbance component, then uses the polarity of transient disturbance power or half-cycle average to locate the voltage sag source. Finally through simulation by Matlab model, this approach proves to be accurate in voltage sag location.