Poisson bright spot is an important experimental phenomenon which be used for proving the fluctuation of light. Based on the Fresnel diffraction theory, the diffraction processes of Poisson bright spot were simulated by a Matlab program in a computer. The change tendency of the Poisson bright spot and its diffraction rings were studied by different wavelength of incidence light, diameter of the disk, and position of the receiving image. This study shows that single dark ring and multiple dark ring can be realized by suitable conditions, which has potential applications in micro-nano fabrications of semiconductors.