Stray light analysis of a three-mirror spatial optical system is presented. The entrance pupil diameter (EPD) of the system is 320 mm, the e?ective focal length (EFL) is 2809 mm, and the field of view (FOV) is 1°×0.5°. Its walls are coated with extinction paint (the absorption coeffcient of which is 97%). The point source transmittance (PST) of the system is thus reduced by up to two orders of magnitude. Moreover, this technique makes it feasible to block the stray light coming from outside of the FOV by increasing the outer baffle length of the system. Adding an inner baffle to both the primary and the secondary mirrors helps not only to block the stray light coming from outside of the FOV but also to decrease the length of the outer baffle. Simulation results show that the PST values are less than 10^-10 when the off-axis angle is larger than 9°. The stray light is also suppressed effectively by placing a glare stop at the first imaging plane of Cassegrain telescope. It is surprising that the PST value is 10^-14 when the off-axis angle is 2° with the placement of glare stop at the first image plane.