The affects of direct sunlights before and after midnights on the imaging quality and thermal stability of an earth observation optical system on geostationary satellite in space were investigated. According to the characteristics of solar heat flux on the geostationary orbit, the energy density of direct sunlight projecting to the optical system was analyzed and a dimensionless parameter named sunlight inhibitory ratio determined by outer baffle′s height ,shape dimension and sun inclination was deduced. It was used to describe the inhibitory ability of outer baffle in the optical system for the direct sunlight and also used as a design principle for the height of outer baffle. The program based on Monte-Carlo method was compiled to solve the sunlight inhibitory ratio of different shape outer baffles in different sun inclination scenarios. In a real design, the sunlight inhibitory ratio was selected to be 0.5 at equinox and the final outer baffle height is about 1.8 times of the east-west axis length of the outer baffle’s bottom side. This way, the incidence sunlight in the optical system from the baffle is three hours. The result satisfies the dimension requirements of satellite payloads.