A satellite-borne solar spectrometer with wide spectral coverage is designed to routinely monitor the long-term variability of solar spectral irradiance (SSI) outside the atmosphere. The instrument employs only one prism with curved surfaces for the dispersion of solar spectrum from 250 nm to 2500 nm, which are scanned simultaneously by several detectors on the focal plane as the prism rotates. The spectral response function (SRF) and spectral resolution of the instrument are also simulated based on Huygens wavelet point spread function (PSF). The tangential aberration of all the wavelengths is smaller than 8 μm at each prism rotation angle within ±2.5°. The wavelength resolution is 0.7～3.5 nm in ultraviolet region (250～400 nm), 3.5～35.0 nm in visual/near-infrared (400～1000 nm) and 28.5～41.2 nm in short-wave infrared (1000～2500 nm). The structure of the system is simple and compact, and the performance is stable and reliable. The spectral dispersion and aberration correction capacity satisfy demand of the long-term measurement of SSI at the top of atmosphere.