The light intensity distribution of Raman Nath diffraction at all levels is obtained by establishing Raman Nath diffraction model and using numerical simulation method. The simulation results indicates that the modulation factor v has an important effect on Raman Nath diffraction phenomenon, and the relative coefficient of light intensity at all levels is clearly changed and become smoothing with the increase of modulation factor. Simultaneously the corresponding experiment is carried out, and the experimental results are consistent with theoretical analysis.