Characteristics extraction and classification of voltage sags is the precondition for choosing the right power quality management measures. Based on the multi-resolution analysis, the Discrete Wavelet Transform (DWT) is simple, fast and non-redundant, but it is generally considered that it is difficult for the wavelet transform to detect the phase shift of the voltage sag. Based on the wavelet-based phasor method, this paper detects and identifies the amplitude and phasor characteristics of voltage sag. Then the source identification features of voltage sag according to amplitude and phasor information of wavelet-based phasor are constructed. Finally, the cause identification of voltage sag is carried out by using the Support Vector Machine (SVM) method. The results show that the proposed method can realize the characteristics extraction and cause identificatin of voltage sag effectively. This work is supported by National Natural Science Foundation of China (No. 51577115).