Abstract: Slots with different depth were made by WEDM in metal sheet of ST12 to simulate slot-type defects, and the Lamb waves were stimulated by pitch catch method. The time domain signals obtained in the plates with and without defect, respectively, were analyzed in time-frequency domain by Short Time Fourier Transform. The lamb wave modes were recognized successfully, and the effects of slot-type defects on Lamb waves was discussed by the time center and the frequency center as digital parameter. The result showed a good corresponding relationship between slot depth and the time center of the lamb wave.