In order to study the effect of substrate on the focusing laser beam in atom lithograph, the simulations of optical potential of Gaussian laser standing wave in the presence and the absence of straight edge diffraction are performed based on the scalar optical theory. The similarities and differences in optical potential between the diffractive Gaussian and non-diffractive Gaussian laser beam are fully indicated by the three-dimensional and cross-sectional figure. The effects of the distance between laser axis and substrate, the distance between laser waist and reflective mirror are fully studied.