Aimimg at the problem of the defects on the surface of components in infrared optical systems, the scattering characteristics of optical components surface under different levels of defects were analyzed quantitatively based on Mie scattering theory. The variation of the bidirectional scattering distribution function(BSDF) on the surface of the optical component was discussed, and then a scattering model of optical components under different levels of defects was established. On this basis, a Gemini telescope was taken as an example, and the self-generated thermal radiation flux and its distribution of the system on the image plane was simulated quantitatively by using the ASAP optical analysis software for the case of the primary mirror under different levels of defects. Furthermore, according to the definition of the effective emissivity, the quantitative evaluation of the stray radiation of the system was carried out. The results indicate that the optical component under different levels of defects not only alter the stray radiation flux and its radiation distribution of the system on the image plane, but also change its effective emissivity, the effective emissivity of the system is 2.19%, 2.34%, 2.46%, 2.59%, 2.72% and 3.08% respectively when primary mirror defect levels under six cases such as 0, I-10, I-20, I-30, II and III. Thus, the stray radiation performance of the system decreases with the increasing of the level of the defects. Consequently, the levels of defects on the surface of the optical component should be strictly controlled in the practical applications.