Abstract: With a large number of sensitive loads connected to the grid, ensuring power quality is a key factor in the safe and stable operation of loads. Voltage sag is the power quality problem with the highest frequency, and fast and accurate detection of voltage dip characteristics is a prerequisite for subsequent voltage compensation and restoration. Due to the long delay time of the conventional DSC method for voltage sag detection, an improved DSC method is proposed in this paper. This method separates the positive and negative sequences of three-phase unbalanced voltages by delaying one sampling period, and combines the coordinate transformation to find out the voltage amplitude and phase of each sequence, as well as finally calculates the voltage-sag characteristics. Through MATLAB/Simulink simulation, the method is able to detect the magnitude of the voltage sag and phase jump value of the voltage sag within 3ms, and the detection accuracy is high, which is a prerequisite for the subsequent fast voltage compensation.