Abstract: In this paper, the modal analysis of Lamb wave was made according to the dispersion properties of its propagation in the thin plateby comparing the two time scales of the signal collected in ultrasonic testing. Based on the dispersion curves of Lamb wave drawn by semi analytical finite element method in the isotropic free aluminum,suitable ultrasonic transducers were selected to obtain modal signal propagation of Lamb wave in aluminum plate in different positions. In the overall time scale of Lamb wave, all the data were analyzed by two-dimensional Fourier transform and the modal and energy distribution would be identified. In the other time scale, the Hilbert-Huang transform was used to separate and distinguish the different modes accurately that were superimposed in a single signal after a reasonable filtering and noise reduction. The results show that we can analyze and compare the signal data of Lamb wave acquired from detection and identify the guided wave mode and get higher time resolution with the above method.