An optical system for CO2 sounder is designed and implemented. The advantages and disadvantages of two dispersive methods, grating and Fourier transfer, are summarized by the contrast of optical systems from different greenhouse gas spectrometers, and the optical system with a large area diffractive grating as a dispersive element is selected for the CO2 sounder. The total optical system includes a fore-optics and a tri-channel grating spectrometer. The fore-optics consist of a non-focal system with double off-axial parabolic mirrors, two dichotic splitters and three focal lenses, in which several measures are taken to reduce stray light. Each channel in the tri-channel grating spectrometer has the same mechanics, and three gratings have the same deviation angle to ensure the high mechanic stability. On the basis of grating equation, a computational formula is derived to calculate the grating parameters at the fixed deviation angle and to determine the parameters of three large area gratings. The lens material is fused silica with a low coefficient of expansion. The diffractive efficiency of the large area grating can be over 90% when it works at a large entrance angle and a large diffractive angle without other order diffractive lights except 0 order and +1 order diffractive lights at selected wavebands. Analysis and experiments on the system show that the system can detect 20 footprints at same time. By setting a 0 order light trap and other stray light removal methods, the stray light in the system has decreased to 10-5, its spatial MTF is more than 0.9(@1.4 lp/mm), and the spectral resolution of the spectrometer exceeds 0.035 nm(@760 nm). Moreover, the large relative aperture(F1.8) increases the ability to collect light. The design results indicate that the optical system can satisfy the technical requirements of CO2 sounders.