Voltage sag has become an increasingly prominent issue in power quality. In this paper, a new voltage sag detection method based on Hilbert transform and morphological filter is proposed to solve the problem of low accuracy and poor realtime performance in voltage sag detection. The basic principle of Hilbert transform for voltage sag detection is discussed. The concept of mathematical morphology is introduced, and the morphological filter is constructed based on it. The selection of filter parameters is given. The performance of this method in voltage sag caused by singlephasetoground fault is analyzed and compared with singlephase dq transformation method. Simulation results show that the method can detect the sag amplitude and phase more quickly and accurately.