Abstract: Proceeding from the definition of the stray light, the source of the stray light was analyzed since the stray light affects the image quality of optical systems seriously. The leading indicator adopted to assess the effect of the stray light on the optical systems, and the mathematic model of point source transmittance and stray radiation ratio were established. The stray light of a Fabry-Perot interference imaging spectrometer was analyzed and calculated with TracePro. It is found that the stray light in optical systems can be effectively restrained and the stray radiation ratio can be effectively reduced by adding a diaphragm in the systems. The optical and mechanical system design of Fabry-Perot interference imaging spectrometer was performed by the aid of the analysis result to eliminate the stray light.