Abstract: The concept and design of infrared hybrid diffractive/refractive athermalized optical system are described. A diffractive optical element is used to athermalize and achromatize the optical system. A hybrid optical system operating at 3.5m～5.2m wave band and the temperature range of -50～80℃ is designed. The system parameters include a focal length of 100mm, F/# 2, and FOV of 3.5, and it achieves 100% cold shield efficiency. The system stray light is analyzed, and the irradiances of the desired light and the stay light are 1.5104W/m2 and 2W/m2 respectively. The image quality achieved in the temperature range of -50℃～80℃ approaches the diffraction limitation. It is applicable to a cooled FPA detector with the format of 320256 and the pixel pitch of 30m.