Abstract: Based on the curve of group velocity-frequency multiply thickness.The time-frequency analysis method based on short time Fourier transform is used for analysis of acousto-ultrasonic signal which propagates in thin plate.The result of acousto-ultrasonic testing experiment of the thin plates of 2mm in thickness makes clear that in the acousto-ultrasonic technique,the excited Lamb waves are multimode Lame waves.Compared with acousto-ultrasonic signal analysis in the thin plate which is based on time field,time-frequency analysis can not only recognize Lamb wave mode but also can analyze energy component of each wave mode.