By the analysis of the scattering situation of the rough surface, it is found that the ordinary metal material scattering energy is centralized in 10°. Based on the scattering characteristics, the method of the outer baffle design is put forward, the baffle rings′ restraint angle is 5° larger than the stray light suppression angle which is calculated based on reflection theory, and the effect of the outer baffle to suppress the stray light is very good. The baffle is designed for a Ritchey-Chirtien (R-C) system and the model is built in the TracePro software for analysis. With the contrast of different models and the designs, the results show that the consideration of the scattering is more accurate, the effect of the outer baffle is better, and the point source transmittance (PST) of the system is lower than others.