Abstract: In view of problem that common detection method of single-phase circuit voltage sags has huge calculated amount and its accuracy of detection result may be affected by unsynchronized signals, a derivation detection algorithm was proposed. The algorithm uses instantaneous voltage value and its derivative to construct linear equation in two unknowns, achieves real-time detection of voltage sags by solving the equation, and solves the problem of huge calculated amount and unsynchronized signals. The simulation results show that the algorithm has little calculated amount, short detection time and high precision, which can meet speedability requirements of detection.