As one of the important technical specifications of diffraction grating, diffraction wave front aberration directly affects the grating resolution. Recording the interference fringe produced from two coherent light beams by use of photoresist is a key process in manufacturing a holographic grating. According to the influence of defocus on collimating light parallelism of reflecting spherical collimating mirror, this paper analyzes the influence of collimating light parallelism on the wave front of holographic grating to improve setting accuracy, minimize defocus of exposure system in holographic grating, and to reduce diffraction wave front aberration of grating. The results of theoretic analysis and numerical simulation show that diffraction wave front aberration of grating is directly determined by the adjusting error of collimating mirror. In addition, by taking three kinds of gratings with different ruling densities for examples, the allowable range of adjusting error of collimating mirror is obtained.