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 mm×55 mm×20 mm. After the optimization, the tangential aberration of all the wavelengths was smaller than 6 μm. 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 2 nm from 600 nm to 1 200 nm 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.