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Four existing voltage sag source location methods based on reactive power variation, slope of the system trajectory, real current component and distance relay are analyzed and compared in this paper. The distance relay method takes advantage of measured impedance as judging criterion and shows the best performance among them. This paper proposes an improved impedance calculation method, which uses the change rate of impedance magnitude and increment of impedance angle before and during the voltage sag event to compare with a reasonable threshold. The improved method has been verified in simulated power systems with different structures, including single-supply, double-supply and ring system. Finally, an application case occurred in an actual city power system in P.R.China is given and good characteristics of the improved method are confirmed.