This paper presented a new detection method of voltage sag feature value, which was extracted from the module matrix of modified Stransform. In order to get the optimal performance, the parameter related to timefrequency resolution was determined on the basis of the flatness and minimal transition time. By means of the optimal timefrequency resolution, the amplitude of voltage sag duration less than 2 cycles could be detected accurately. The difference vector of base frequency was employed to detect sag duration so as to overcome the flaws of exiting Stransformbased methods, whose effects were unsatisfied when there were strong noise and no amplitude abrupt. A comparison between Stransformbased method and modifiedStransformbased method was made, and the results showed that the later was superior to the former in magnitude detection and duration localization.