Locating the voltage sag source is to determine on which side of a monitoring device the voltage sag originates. This paper takes the analysis of power grid faults as an entry point and applies the port network theory, and then any complex power network at both sides of voltage sag source monitoring devices can be equivalent into two active single-port networks respectively. Grid faults are regarded as external disturbances of “active single-port network”, meanwhile the internal resistance of the “active single-port network” will be measured and calculated. Based on the sign of internal resistance in these two active single-port networks, the voltage sag source location relative to the monitoring devices can be identified accurately. If the sign of internal resistance is positive, the location of voltage sag source is opposite to the reference direction, that’s upstream. If the sign of internal resistance is negative, the location of voltage sag source is consistent with the reference direction, that’s downstream. Calculation method of internal resistance is given. The proposed method has a solid theoretical basis. Simulation results show that this method can locate voltage sag source with an accuracy of 100% in power networks with various topologies, such as radial or non-radial power systems as well as the network adopted neutral grounded effectively or ineffectively. It is a practical and effective method for voltage sag source location.