In order to realize the detection of voltage sag fast and accurately, a novel method for detecting voltage sag by a modified incomplete S-transform is proposed. The main idea of the method is as follows: first of all, the fast Fourier transform of the voltage signal sampled from the power network is made to obtain the main frequency points of the signal. And then, the modified S-transform of each frequency point is performed to attain the components of main frequency characteristic quantities. Finally, an implementation of the detection of the characteristic quantities of voltage sag is carried out by the components. The principles of the proposed modified incomplete S-transform and the choice of the parameters are described, and then the extraction method of the features of voltage sag is stated in detail, and finally the practical signals are simulated. The obtained results show that the characteristic quantities such as the amplitude, duration and a sudden change of the phase of voltage sag can be extracted by the proposed method, and that it is characterized by high precision, less computation and good anti-interference performance.