Abstract: The thermoelastically generated ultrasound in double layer materials was numerically simulated by finite element method(FEM), and the stress field on top and bottom surface was obtained. Then, the frequency signal from the waveform of stress was obtained by using FFT, according to the group velocity of the ultrasonic packet so that the modes of Lamb wave in the materials could be found out. The results showed that three kinds of modes of Lamb wave influenced by the substrate were fount in aluminum plate of the double layer materials. The results could provide a new approach for the research of the laser induced ultrasound in composite material.