In view of the duration of voltage sag detection problem, which put forward a new method of using the Hankel matrix singular value decomposition (SVD) theory to detect the voltage sag begin-end time , and applicability simulated analysis the different drop amplitude, harmonic and noise under three kinds of jamming, and propose to contain the noise voltage drop choosing appropriate SVD components for signal reconstruction, evaluate the SVD in the reconstructed signal voltage dro-p begin-end time. By Matlab7.0 platform simulation validation of the proposed method, the results show that the detection method principle is simple, testing precision and strong applicability.