In order to realize the sunlight observation with each wavelength, aiming at the narrow spectrum test range and low spectral resolution weakness of solar radiation observation instrument, the optical system design scheme was proposed which can realize nanometer resolution based on applying spectral measurement technology. Combining with the characteristics of the sunlight and the service environment of the solar radiation observation instrument, the collecting light system was designed basing on fiber and cosine corrector. According to spectral range and spectral resolution, the spliting light system was designed with multi-channel and manometer resolution. Experimental and design results indicate that the spectral resolution precedes 20 nm and realizes the sunlight observation. It can satisfy the instrument requirements of wide spectrum and nanometer resolution.