Fourth order Runge Kutta method was proposed to solve the acousto optic couple wave equation derived from the multiple scattering theory. Bragg diffraction efficiency varying with the key parameters was numerical simulated. The relation between the Bragg diffraction efficiency and the incidence light which was deflected from the exact Bragg incidence angle was analyzed. The optimal parameters selected according to the Bragg diffraction efficiency was discussed too. The numerical simulation result showed that, when Q=4π, the Bragg diffraction efficiency was 97%, this was coincident with the traditional characteristic length.