Voltage sag has been considered the most common power quality problem. Fast and accurate detection of typical characteristics is an important prerequisite for compensation and governance of voltage sag. In this paper, a voltage sag detection method based on derivative αβ-dq transform and improved morphological filtering is proposed. Based on the derivative αβ-dq transform, this method used the improved morphological filter to achieve accurate and fast detection of voltage sags. The improved morphological filter improved the real-time performance by modifying the corrosion operation and expansion operation. According to the characteristics of a voltage signal and morphological filter, the signal-to-noise ratio of filtering signal is taken as the fitness function to find the filter parameters, so that this method can more accurately and reliably extract the voltage sag characteristic quantity. Finally, the simulation analysis verified the accuracy and reliability of the method. The results show that the method can detect the characteristics of sag depth and phase jump quickly and accurately, which is suitable for voltage sag detection and treatment device, and has practical engineering application value.