Abstract: Due to its excellent time-frequency combination properties, S transform has got great attention in the field of signal processing. This paper analyzes the time-frequency methods and points out the importance of the S transform algorithm’s applying in the field of communication signals’ reconnaissance. Based on the study of basic principles of S transform, the commonly used algorithm of discrete S transform and its disadvantages are analyzed in this paper. The improved discrete S transform fast algorithm is proposed to reduce the computation of discrete S transform as well as achieve the fast discrete S transform operations. To examine and certify the effectiveness of the proposed algorithm, the simulation and the compare of the performance among the improved discrete S transform, the traditional discrete S transform and CWT are preceded. The result shows that the amount calculation of the improved discrete S transform fast algorithm is less one or more orders of magnitude than the traditional discrete S transform and CWT. Simulation and performance analysis proved the effectiveness to improve algorithm of discrete S transform. The improved algorithm has great significance to the engineering of the communication signal’s fast reconnaissance.