Based on the method of Collins diffraction integral formula and expanding the rectangular function into a sum of complex Gaussian functions,the effect of smoothing by spectral dispersion (SSD) considering the hole size used in spacial filter is studied,and the method of the selection of hole size in view of aberration is given in the article using the statistical optics model raised by K.R.Manes et al.. Numerical simulation show that the holes now used in experiment are small for SSD,fluence beam contrast without holes raise from 22％ to 36％,and calculate the needed pinhole size for SSD.