Abstract: MARC finite element software was used to obtain the single mode A0 and S0, which was achieved by loading displacement based on the distribution of different mode wave structures in finite element model. In addition, the propagation process of A0 and S0 mode wave in 2mm sheet was simulated, and the diagram between the reflection coefficient of defect echo and defect size was also drawn. And then, the simulation results were verified by the practical experiment. The results showed that the excitation of single mode Lamb wave could be achieved by the displacement loading of wave structure. It was feasible to estimate the defect size combined with finite element method. The sensitive degree of different vibration modal on defect size was different. The A0 mode was sensitive on defect size, whereas the defect size was unable to be identified by S0 mode.