Grating scatter is an important performance index for plane grating, and its measurement is a difficult problem for grating research field. In order to achieve stray-light of the instrument itself to be less than 10-8 so that the grating scatter can be accurately measured, stray light in diffraction grating monochromators has been studied and an opto-mechanical model of measure instrument for plane grating is designed, which works in parallel light environment and based on the scalar diffraction theory and classical Fresnel-Kirchhoff diffraction theory. Main pathway of the instrumentation’s stray light is calculated and analyzed by the simulation of stray-light testing equipment with ASAP software. Accordingly, four stray-light suppression structures including blocking rings, vanes, aperture stops and light traps are proposed to reduce instrument’s scattered light and grating’s multiple diffraction. Finally, contrast analysis of the instrumentation’s stray light is made before and after adding stray-light suppression structures. Simulation and analysis results show that the max of instrument’s stray light reduces from more than 10-6 to less than 10-8 after adding stray-light suppression structures and has met the stray-light testing equipment’s design requirements whose goal is to realize accurate measurement greater than or equal to 10-7 for grating scatter with grating’s groove density changing from 300 to 3 600 gr·mm-1. The research methods and results above will provide a theoretical basis for the research and development of the stray-light testing equipment for plane grating.