With the development of technology, the requirements of electrical equipment for power quality are higher. Voltage sag problem is getting more attentions, where accurate detection of voltage sag is the premise of compensating the question of voltage sag. In this paper, employing the filter function of the structure adaptive notch filter and frequency adaptive method can accurately detect the fundamental phase voltage signal and the quadrature signal under frequency changing. Thus, it can obtain the amplitude and phase information of the voltage, and reaches the voltage sag detection. The method is simple, and eliminates harmonic influence, and adapts frequency changing. Simulation and experimental results show that voltage sag detection method employing the adaptive notch filter can accurately obtain the voltage of each phase voltage sag information.