In this paper

by making use of the equivalent relation between free-space fresnel diffraction illuminated by the spherical wave and fractional fourier transform and the correlation between long distance and short distance diffraction

we proposed a new algorithm for Fresnel-diffraction based on the Wigner transform and simulated the conclusion by computer. Compared to traditional algorithm

the new one is not only more simple but also proposes a new way to study the distribution character of light space.