Stray light analysis in optical system is a key means to improve the signal to noise ratio and ensure the measurement accuracy. Based on the optical system structure and working mode of space-borne spatial heterodyne spectrometer for monitoring greenhouse gases, the outer and inner light shield was designed. With the Tracepro model, the stray light of oxygen channel(758-768 nm) in the system was simulated and evaluated, then (PST,Point Source Transmittance)of the system was calculated, PST values were less than 5×10-7 at the offaxial angle of 30°, and the stray light coefficient was less than 4%. The obtained PST value and the stray light coefficient shows that the light baffle and other mechanisms have suppressed the stray light effectively, and the stray light suppressing level of the system satisfied the requirement of design.