Light scattered from the imperfect surface of a primary mirror in a reflective optical system will result in the in-field stray light and reduce the image contrast. This paper presents a simple method to process the scattering of the primary mirror.A Harvey Bidirectional Scatter Distribution Function(BSDF) model is used to characterize the angular distribution of scattered light from the mirror, and the scattering distribution at the focal plane of the primary mirror is derived. The characteristic radius centered by the image point can be achieved when the intensity of scattering equals that of diffraction. It is found that for the given optical system, the characteristic radius only depends on the scattering characteristic of the primary mirror. The intensity distribution at the focal plane is divided into two regions by the characteristic radius. The intensity of diffraction is larger than that of scattering intensity,which makes the dominant contribution to the saturation effect when the distance from the image point is less than the characteristic radius, conversely, the scattering is the main factor to the saturation effect when the distance from the image point is larger than the characteristic radius. For a primary mirror with a focal length of -757 mm, an radius of 115 mm and the rms roughness of 10 nm at the wavelength of 550 nm, the characteristic radius is 0.843 mm approximately. These conclusions are conductive to the analyzing and explaining of the saturation effects in an optical system irradiated by a intense light.