In order to improve the spectral resolution and spectrograph efficiency of LAMOST-HRS(Large Sky Area Multi-Object Fiber Spectroscopy Telescope-High Resolution Spectrograph), and set up the methods to analyze stray light in the conceptual design stage, the modeling and analysis of stray light without measuring BSDF were studied. First, the Harvey scattering model was constructed by calculating the key parameters according to the roughness measurement data. Then the optical surface was observed by microscope, and the maximum partical diameter was obtained by MATLAB image processing, and the particle pollution scattering model was built. The the coatings, optical element and mechanical structure of the spectrograph were imported. The mechanical structure was simplified to improve the efficiency of the analysis. Finally, the background of the stray light was estimated, and the path and composition of the stray light were analyzed. The results show that the LAMOST-HRS stray light is mainly caused by the scattering of optical surface. The stray light accounts for 2.55% of the total illumination of the CCD surface, and the signal-to-noise ratio is 16.01 dB, which meets the design requirements.