Abstract: It is very important to study the interaction of fundamental single mode and defects in the area of middle-range and long-rang lamb waves inspection. Modularization programs were designed to load the displacement on the FEM modal in the way of mode wave structure using the ANSYS Parameter Design language, obtained the pure single S0 and A0 mode was obtained. It proved the correctness and reliability of this method. Comparing the obtained waveform of loading the wave structure in certain range frequencies of bandwidth and in the centre frequency of excitation signal respectively, it presents that when the wave structure variety is not obvious, usually less than 2 MHz·mm in the range of fd, the wave structure of centre frequency can replace the summing wave structure of a range of frequencies .