In order to solve the stimulated Brillouin scattering (SBS) damage by reflected diffraction of fused silica grating with large aperture, we establish the design and analysis model of grating, the reflected diffraction angle, focal length and efficiency from 1 to 6 orders reflected diffraction of the grating are calculated. The gain and intensity of SBS with modulated laser and original laser are analyzed based on SBS principle, and the reliability of the analysis is verified by an engineering example. The experimental results show that the SBS intensity is large, the grating element is seriously damaged and its service life is short when the phase modulation is not applied. Moreover, when the phase modulation is applied, the SBS intensity is less than the damage threshold of fused silica and the service life of grating element increases evidently. In the actual high power laser system, phase modulation must be applied to suppress SBS damage of grating reflected diffraction.