Summary This paper briefly introduces the existing methods for voltage sag source location and theoretically analyzes some reasons why high accuracy cannot be achieved by using these methods. Based on the redefined voltage sag source location “reference direction” and the analysis of voltage sag source's influences in power network, it is pointed out that observing from disturbance source point to both sides of monitoring services, any power network can be equivalent to 2 active single-port networks and the networks can be equally replaced by “Thevenin's equivalent circuit.” By judging the sign of the internal resistance in Thevenin' s equivalent circuit, the origin of voltage sag disturbance can be easily identified. The calculation of internal transient resistance is given based on the application of space vector theory. This method can locate voltage sag source with an accuracy of 100% verified by simulation experiments. It is a very practical method that is not only applicable in radial and nonradial power systems but also in networks of neutral grounded effectively or ineffectively.