The temperature characteristics of diffractive optical element are analyzed. An infrared athermal imaging optical system at MWIR bands and 5.2° field of view is designed by using diffractive optical element. Design results show that the image quality of the system approaches the diffraction limit in the working temperature range of -55 ℃ to +80 ℃, and the entire field of modulated transfer function (MTF) at 33 lp/mm is more than 0.5. The maximum diameter and the total length of the optical system are 110 mm and 90 mm respectively, which meets the technical requirements.