The accurate identification of voltage sags helps to solve the problem of voltage sags and to deal with the disputes of supply and use just.In this paper, a new identification method of voltage sag source in distribution network is proposed, which combines fast Fourier transform (FFT) and a shape-based weighted dynamic time warping (SWDTW). First, the characteristics of voltage sags caused by single sags and composite sags are analyzed. Secondly, the characteristics of the signal harmonic are analyzed by FFT method, and the feature waveform library is constructed and divided. Then, in view of the shortcoming of traditional dynamic time warping (DTW) method,a shape-based weighted dynamic time warping (SWDTW) method is applied to match the one-dimensional waveform data. Finally, the simulation shows that the proposed method can effectively realize the identification of voltage sags and provide strong support for the effective control of voltage sags and the resolution of related disputes .