The precise location of voltage sag disturbance source relative to monitoring site helps to clarify the accountability of the responsibility either by the customers or the suppliers. In order to precisely locate the voltage sag disturbance source, a new location method of disturbance power has been proposed based on single fundamental frequency cross S-transform. First, the voltage and current signals, which are collected by the monitoring sites, are transformed by single fundamental frequency cross S-transform, thus obtaining the fundamental vectors of the voltage and current. Next, with the fundamental vectors of voltage and current transformed by single fundamental frequency cross S-transform, the instantaneous apparent power can be extracted from the model, and the instantaneous apparent disturbance power can be obtained. Finally, locate the source of voltage sag disturbance by adopting the disturbance power criterion. An analysis has been made of the short circuit fault, induction motor starting and transformer inrush current and fault clearance of four typical voltage sag source due to modeling and simulation, as well as the eight monitoring points, which result verifying the accuracy and effectiveness of the proposed method.