Abstract: It is multimode Lamb waves that compose the Acousto-ultrasonic (AU) signals when the thin plate is tested by AU method. Because of the complication of Lamb waves, AU method avoids the further analysis for signal components, and constructs so-called'Stress Wave Factor' instead to measure the dissipation of the energy. It is very hard to recognize the signals with both multiple modes and dispersion. In this paper, time-frequency analysis is used to separate the AU signals and only a single measurement is required. By combining the pseudo Wigner-Ville distribution with reassignment technique and feature extraction, portions of dispersion curves of Lamb waves can be obtained accurately.