Abstract: Aiming at ultrasonic Lamb wave detection of defects in multi-layer plate, the dispersion curve and displacement wave structure of multi-layer structure of steel-aluminum-copper plate were calculated and analyzed. A defect detection method in multi-layer plate structure was proposed and verified by numerical simulation. The numerical calculation shows that when the low-frequency Lamb wave of 0. 2 MHz is used for detection, the defects of each layer in the multilayer plate produce reflected signals. When the A0 mode detection of 1 MHz high-frequency Lamb wave is adopted, only the defects in the copper plate layer produce the reflected signal, and when the S0 mode detection is adopted, only the defects in the steel plate layer produce the reflected signal. Therefore, according to the analysis results of the frequency dispersion curve and wave structure of multilayer plates, Lamb waves with appropriate frequency and mode can be selected to detect the defects of specific layers in multilayer plates.