Voltage sag is a serious power quality problem which frequently occurs in power system, but traditional voltage sag detection methods still have deficiencies in rapidity and accuracy. So here propose a fast single-phase voltage sag detection method based on morphological filter combine with differential coefficient αβ-dq transform method. The principle of derivative αβ-dq transform method has been analyzed in detail, then choose appropriate morphological filter by comparing the ability of morphological filter with different structure element in order to filter harmonic and noise which generated by power system, so the accuracy of algorithm could be increased. Simulate and analyze this method by using computer software and then further validate this method by using it to analyse data collected by a simulation voltage sag data gathering module built in lab. All those consequence show that this method can detect the amplitude, the onset time and the phase of voltage sag quickly and accurately.