The optic characters of canopy reflectance spectral and the optic principle of non-destructive nitrogen monitoring were analyzed, and then the optic parameters disturbing the monitoring precision of the optic system were put forward, which included the uniformity in image optic field, the wide object field with short distance of survey, the operation dithering and the change of distance of survey. The scheme of optic system frame design, the key optic organs choosing and its optic parameters matching were proposed subsequently. Finally, the causes and control methods of the optic system measurement error were analyzed and the sample product was designed and manufactured. 