Abstract: A two-dimensional plain strain finite element model was built, on the basis of investigating the ultrasonic wave exciting and propagating on the model, using Ultrasonic spectral method to sizing different depth of surface-breaking cracks. In order to show the scattering and complex mode-conversion clearly, the resuls were presented in the form of snapshots. And then, on the basis of spectral analysis to the transmission signal to obtain the relationship between slot depth and the absorption frequency.