In order to measure the absorption spectrum, whose radiation wavelength range is between 1 591 nm to 1 621 nm (near infrared band)，a kind of high spectral resolution spectral detector is designed for measuring the near infrared CO₂ absorption bands, and the instrument underlying is in progress. Compared with existing optical systems applied to spaceborne hyperspectral imagers abroad, a simple planar reflection grating structure is selected for beam splitting. The whole detector is consisted of a fore optics system and a spectral imaging system. To fit the big opposite aperture (F1.8) and small IFOV, Petzval construction is applied to the fore optics system. The system’s simulation and optimization is completed by ZEMAX optical design software, which shows that the optical design completely and feasibly meet the application requirements. System realize the high spectral resolution of 0.1 nm, and the Modulation Transfer Function (MTF) of both the center and the edge wavelength are higher than 0.65 at the characteristic frequency of 20 lp/mm.