A stray light removing mechanism is designed for improving image contrast and image deblurring in a spaceborne camera. The spaceborne camera researched in this paper is based on on-axis Three Mirror Anastigmatics(TMA), and the stray light removing mechanism consists of inner and outer baffles, field stop, Lyot stop and blocking board. Cassegrain system is taken as a reference in the CAD modeling for design of inner and outer baffles and the random light Moente-Carlo simulation method is adopted to evaluate the effect of stray light removing. Moreover,the Bidirectional Scattering Distribution Function(BSDF) for describing the scatter character of surfaces, choice of key surfaces and the establishment of model are introduced and the stray light ratio and Point Source Transmittance(PST) curves are acquired by computer analysis.Results show that the stay light ratio is below 0.35% when stay light incidents at +10° meridional direction and the PST is at the level of 10-6 in 0~20°. These results can offer a good reference for the further optimization design.