In order to realize the accurate identification of various voltage sag sources, a method based on dynamic time warping (DTW) is proposed from the perspective of waveform similarity detection. First of all, the simulation model is used to analyze the characteristics of basic voltage sags and its different types after the transmission of transformers, the relationship among voltage sag types, causes and transformer types is established. Then, DTW is used to match the measured voltage sag and the matching library, which is formed by nine kinds of voltage sag. Simulation results show that the method can identify various type of voltage sag of power grid and is accurate and effective in engineering application. This work is supported by National Natural Science Foundation of China (No. 51477029) and Science and Technology Program of State Grid Jiangsu Electrical Power Company (No. 5210EF14001R).