Spaceborne CO₂ imaging spectrometer has become one of the important means to monitor global greenhouse gas changes due to its advantages of spectrum integration, high spatial resolution, high time resolution, non-contact and long-term monitoring. In order to solve the problems of large incident slit, low signal-to-noise ratio and poor imaging quality in large field of view, a new scheme was proposed for the initial structure design of the optical system of Offner imaging spectrometer. The scheme was based on the tangent of meridian ray and arc sagittal ray emitted at the 30 mm slit at the central wavelength to improve the utilization rate of incident light in the whole spectral range. Using optical design software, an imaging spectrometer with F number of 2.5 and spectral system resolution of 0.1 nm was designed in the range of 1594-1619 nm. The design results show that the root mean square (RMS) radius of the sequence diagram is less than 5 μm, and the modulation transfer function of the system at 33 lp/mm is better than 0.7. In addition, the system uses pixel merging (i.e. extended pixel) method to further improve the detection intensity of spectral signals. The design scheme satisfy remote sensing detection requirements of large field of view, high spectral resolution and high signal-to-noise ratio for spaceborne CO₂ imaging spectrometer.