Space-borne differential optical absorption spectrometer is used for atmospheric trace gas distribution monitoring through acquiring high accuracy ultraviolet/visible (UV/VIS) radiation scattered or reflected by air or earth surface. To achieve the goals of large view angle, wide UV/VIS waveband detection, high spectral resolution and compact structure in space, a fore optical system is designed. It includes fore-optics telescope and relay optical system. To detect viewing field at 114° in cross-orbit direction, fore-optics telescope with two pieces of off-axis spherical mirrors is designed. Relay optical system consists of relay mirror, color-separation filters, and relay lens, which separates the incident light into four spectral channels covering 240~710 nm. Then we import each beam of light into corresponding spectral imaging channels respectively. The design result shows that the fore optical system works efficiently inside the field of view for both UV and VIS waveband, providing good imaging quality. That makes it possible to implement high spectral resolution of subsequent systems. The fore optical system has more compact structure and lighter weight comparing with other products. It can meet the requirement of spaceborne and airborne platforms without using a scan mirror.