Abstract: In this paper,a hybrid boundary element model combining traditional boundary element method with Lamb wave normal mode expansion technique was introduced,and the new method was used to solve the scattering problem of A0 Lamb wave by surface breaking rectangular defect in aluminum plate.Under variable frequencies,the reflection and transmission from the crack were calculated and mode conversion characterization was obtained.The snapshots of A0 Lamb wave propagating in the plate and the scatter from the crack were presented.The method can be used for reference in nondestructive testing of plate-shaped structure using Lamb waves.