In view of the fixed window function of S-transformation and the problem of detection accuracy when detecting the start and end moments of disturbance signal, amplitude variation, and phase change, this paper introduces two tuning factors in S-transformation to obtain an improved one. S transform, and the inverse transform of the improved S transform to get the TT transform, the improved S transform and the TT transform are applied to the three sources of the disturbance that mainly cause the voltage sag, and on the MATLAB to the three a kind of temporary disturbance source signal was detected and analyzed. The MATLAB simulation results show that the proposed method has better effect and higher detection precision than the S-transform method in detecting voltage sags, which lays the foundation for better extraction of the characteristics of sags and identification of temporary sources.