A scattering model of the optical component substrate under different levels of defects has been established based on Mie scattering theory, and then the scattering characteristics of the replication of substrate defect have been analyzed quantitatively when the particle contamination exists on the surface of the optical component. On this basis, a R-C optical system has been taken as an example, and stray radiation characteristics of the system have been simulated and analyzed by using the ASAP optical analysis software for the case of the primary mirror substrate under different levels of defects. Furthermore, according to the calculation method of the signal to noise ratio(SNR), the SNR of the system has also been carried out and analyzed. The results indicate that the SNR of the system decreases with the increasing of the defect levels of primary mirror substrate for a given radiant temperature of the sky background, and the influence of the SNR decreases with the increasing of the radiant temperature of the sky background for a fixed defect level of primary mirror substrate. Finally, for the primary mirror particle contamination of 300 grades (the particle surface coverage is 0.03%) and its substrate defect levels under five cases such as I-10, I-20, I-30, II and III, the relative SNR(relative to the ideal primary mirror) of the system are 0.932, 0.920, 0.906, 0.832 and 0.807, respectively when the radiant temperature of the sky background is 200 K. Thus, when the optical properties of components become worse, the levels of optical component defects should be strictly controlled within II level to ensure the effective detection of the weak signal with low SNR.