Abstract: The ultrasonic Lamb wave method is used to detect and study the effective identification of incomplete penetration in thin-walled cross laser welding structures. The finite element analysis method is used to simulate the effects of A0 and S0 mode Lamb waves in thin walls and the defect. The relationship between defect size and echo reflection coefficient is predicted. On this basis, through simulation combined with experimental research, the incident point and excitation angle of different modes of Lamb waves are determined. At the same time, the feasibility of using A0 and S0 modal waves to identify incomplete penetration of 1.9 mm thick thin plate cross laser welding structure is discussed. And the relationship between Lamb wave reflection coefficient and defect size is obtained. The results show: The trends of simulation research and experimental results have good consistency. With ultrasonic Lamb wave detection, A0 modal waves can effectively detect incomplete penetration in the cross welded structure and can characterize the size of defects to a certain extent; While S0 modal waves are not easy to identify incomplete penetration in the structure.