Classification of voltage sags is the precondition for choosing the right power quality management measures. This paper proposes a new voltage sag identification method based on the module matrix of generalized S-transform. It firstly determines the regulatory factor of improving the S-transform by the main frequency constituents of sags and then calculates the modulus matrixes of the S-transform to form the standard templates. The sags are classified through the analysis of the similarity between the specific region of the matrix of the test sags and the standard templates. In the process of similarity calculation, for enlarging the similarities and differences among standard templates, the idea of calculating similarities at different frequencies is proposed, thus making the differences maximized. This method sufficiently excavates the characteristic differences among sags; no extra classification process is needed. Simulation and actual data analysis show that this method is easy to be implemented and has high accuracy and noise robustness.