Aiming at the shortcoming of low accuracy and poor realtime performance of voltage sag detection method, a new method based on improved dq transform and sequential morphological filter is proposed. Basic principle of improved dq transform for voltage sag detection algorithm is researched, the concept of order morphology is introduced. And thus, the morphological filter is designed, and the method of selecting filter parameter is expounded. By setting different signal noise ratio and leading angle, the simulation results show that the new method can detect the sag under ideal condition and nonideal condition. It is proved that the new method can detect the eigenvalues and the stopping time of the sag more efficiently and accurately.