Abstract: In order to get the theoretical velocity of each acoustic wave mode on the plate, Newton tangent method is used to solve the Lamb dispersion equation. When solving phase velocity, the frequency and thickness of the plate are independence in the calculation, and when solving the group velocity, the method of derivation for implicit function is used. The whole calculation process is simple, efficient, and easy to grasp, with which the difficulty of solving the equation by programming is overcome. Then, the theoretical result is proved by an experiment of AE instrument. The data are contrasted each other. The theoretical velocity of the extensional wave mode is 400m/s higher than the experimental result, while other velocities are similar.