The principle and design method of employing the thermal property of diffractive optical element to athermalize optical systems are expounded. A method utilizing the best image plane to determine the optimum correction coefficient in order to optimize the performance of optical systems in required temperature range is described. The optical prescription and evaluation of an athermalized and achromatized IR system in temperature range 20 ℃～50 ℃ and 8 μm～12 μm waveband are given. The system has simple construction and reliable performance in required temperature range.