Since the low time-frquency resolation and heavy computation of S-transform when comes to detecting voltage sags,the modified S-transforn(MST) algorithm was established.Voltage sag parameters are obtained with MST in a higher resolation and speed.Firstly,the standard of adjusting factors in MST are given, and the values of adjusting factors are given by experimental analysis;Secondly,according to the distribution of the adjustment factor, the adjusting factor of piecewise is proposed.Finally,the adaptive detection method is established based on the function of adjusting factor.The simulation and test results show that the proposed method provids results with higher accurcy in time-frquency anaysis,and is appropriate for accuate and rapid detection of voltage sags.