In order to deal with the detection problem of the voltage sag features, this paper introduces two kinds of time-frequency methods, the STFT and S-transform, then the two methods are compared and analyzed. The different widths of the window function in the STFT is selected to decompose the same voltage sag signal in time-frequency. And the effect on the test results of different width is analyzed. This paper proposes to use time-frequency isopleth to position the start and end time of the sag, use the fundamental frequency amplitude curve to detect the sag amplitude, and use the phase hopping curve to determine whether the phase hopping happened when the sag occured. The simulation results show that the smaller the window is in STFT, the more accurate the test results are; compared with STFT transform, S-transform test results are more accurate and has high anti-noise ability, which is helpful to power quality management.