Poisson bright spot is a classic optical phenomenon caused by light diffraction，which is used as a typical demonstration experiment in the college physics for proving the wave character of light．The diffractions of circular screen are numerical calculated． And the influences of several parameters are systematically analyzed，including wavelength l，radius of circular screen Ｒ，distance between circular screen and receiving screen z，and roughness of the edge of circular screen．The results show that the number of Fresnel half-wave zones k is an important parameter．For the values of k that supporting high levels of Poisson bright spot intensity and contrast，the smaller k is supposed to benefit the demonstration．