Abstract: A small-size satellite-borne solar spectrograph with wide spectral range was designed to monitor the Solar Spectral Irradiance (SSI) outside the atmosphere. The instrument used a modified Czerny-Turner spectral system. A beam splitter was used to split the spectral coverage into two parts: the first order spectrum ranges from 1 200 nm to 2 400 nm and the second order spectrum ranges from 600 nm to 1 200 nm. Two linear array detectors were used to get the whole spectrum at the same time for a direct-reading spectrum. The dimension of the optical system was about 80 mm55 mm20 mm. After the optimization, the tangential aberration of all the wavelengths was smaller than 6 um. The Spectral Response Function(SRF) was simulated based on the Point Spread Function(PSF) which obtained by direct integration of Huygens wavelets method. The spectral resolution is superior to 2nm from 600nm to 1200nm and 4 nm from 1 200 nm to 2 400 nm by analyzing the spectral bandwidth. The structure of the system is simple and compact with a stable performance. The instrument is applied to the measurement of SSI on the satellite.