Adhesive lap joints between composite and metal plates have been widely used in industrial fields including the automotive industry, marine manufacturing and aerospace engineering. Low quality of operation, harsh environment, adhesive aging and other disadvantages may lead to disbonding. To assess the disbond contour at an adhesive interface, this study proposes a detection method based on the phase velocity variation of Lamb waves. First, the dispersion curves of Lamb waves in both single-layer and bonded multi-layer areas are acquired using the semi-analytical finite element (FE) method. Subsequently, numerical models of Lamb wave propagation in intact and disbonded joints are established. Due to the difference in phase velocity between relevant modes, the phase difference of Lamb wave between disbonded and intact joints is quantitatively linear with the disbond length under specific excitation, which is verified by the simulated signals based on FEs. Then, a probabilistic reconstruction algorithm based on phase delay is employed to localize the disbond center. On this basis, the edge points of the disbond are acquired, and the convex envelope of these points is sketched for disbond contour estimation. As a result, both the location and shape of the disbond can be obtained, thereby providing information for subsequent assessment. The experiment is carried out on an adhesive lap joint specimen composed of an aluminum plate and a quasi-isotropic carbon fiber reinforced plastic laminate, and the results demonstrate the effectiveness of the proposed method.