In the fabrication of the holographic diffraction gratings, the scatter light which is produced by the reflection of the granule in air or the diffraction defect of the optical apparatus will be recorded simultaneously in the resist. Those noises not only reduce the diffraction efficiency of gratings, but also give rise to the scattered light in diffraction. So an appropriate spatial filter should be used to stop the higher spatial frequencies corresponding to the Gaussian beam. Because the diffraction effect after the pinhole aperture can demolish the wavefronts of laser, it is the most important to choose the optimal pinhole for adjusting the waist of the laser. On the basis of standard scalar diffraction theory, the field amplitude is given by the convolution calculation, the phase distortion of diffraction wavefronts is also analyzed, and the lower limit ratio of the aperture size with respect to the beam radius is given when the phase transitions are further away from the center. Using the energy conservation of Gaussian beam and comparing the characteristics change of diffraction beam, the effect of diffraction is presented and the upper limit ratio of the spatial filter is given by the critical condition of the diffraction. The conclusion shows that if the ratio of the aperture radius to beam waist is between 1.52 a/02.2, the phase is invariable in the area of the exposal, and the power transmitting the spatial-filter pinhole aperture is slightly more than 99%, so spatial filter performs its function only in this range of the radio.