Abstract: Proceeding from the wave equation, the coupled wave equations for Bragg acousto-optic interaction of the traditional diffraction sources were deduced to ensure the quality of the signal light beam in the synthetic aperture lidar testing,. According to the different requirements of each testing on the beam quality, the spatial three-dimensional distribution of the light intensity in the diffraction field is analyzed by the aid of the finite difference method. It is found that uneven distribution of the acoustic amplitude affects the intensity distribution in the acousto-optic diffraction field and the inhomogeneous intensity distribution results in the difficulty of information source detection in the experiment. By means of the research of the intensity spatial distribution, the effective clear aperture was increased to 3 mm and the problem of the beam diffraction quality of the acousto-optic modulator was solved. The improved beam quality meets the requirement of the experiment. This method is applied to improving the working parameters of acousto-optic devices.