Dispersion curves are of great important to Lamb wave in ultrasonic nondestructive testing. The narrow band pulse was employed to generate Lamb wave. Rayleigh Lamb equation was analyzed and a numerical calculation method for Lamb wave dispersion curves was presented based on matlab, by which the phase velocity curves and group velocity curves of Lamb wave in composite material plate were plotted respectively. It was of significance for the use of Lamb wave in nondestructive testing.