Based on the design analysis and calculation of multi-layer diffractive optical elements in medium and long wavelength infrared bands,an infrared optical system with a focal length of 200 mm is designed.The transmittance in medium and long wavelength infrared bands is greater than 80%,the imaging effect is good and meets the requirements of non-thermal design.Double-layer diffraction technology is adopted,and materials such as germanium,zinc selenide and zinc sulfide are used to achieve the diffraction goal with high efficiency and improve the optical transmittance of the two bands.The optical system structure of secondary imaging is adopted to realize optical small-diameter design.The optical transfer function can achieve the requirements of non-thermal imaging in the environment of -55 ℃ to +71 ℃,thus achieving the design purpose.