Laser-generated ultrasonic wave is effective for evaluating near-surface elastic property. Taking account of the gradient changes of the near-surface elastic property caused by surface heat treatment or laser shock peening, a finite element method is developed to simulate the laser thermo-elastic generation and the propagation of acoustic waves in the system of multiple gradient layers of the substrate. The number of discretized layers has an effect on the generalized wave modes. Two systems with the surface layer softened and the surface layer hardened substrates are respectively considered. And the two-dimensional fast Fourier transform method has been used to obtained the phase velocity dispersion of different modes. The conclusion could be useful to the further study of the near-surface elastic property.