Abstract: Ultrasonic Lamb waves characteristic of multi-modes and dispersion caused the signal to be too complex to identify. In order to solve the problem, combining with the main design parameters of electromagnetic acoustic transducer(EMAT) and the characteristic equation of Lamb wave, the exciting equation and curve were deduced. Through choosing the appropriate operating point on the curve to depress the influence of multi-modes and dispersion, an optimal excitation mode of electromagnetic ultrasonic lamb wave was proposed. Zero-mode Lamb waves validity in application was analyzed by this method, and verified by experiments.