In order to realize the detection, identification of long-distance infrared radiation targets. Through the research and analysis of the model and theory of harmonic diffraction, free-form surface and off-axis three-mirror optical system, we propose a design scheme of off-axis triple-mirror dual-band infrared imaging optical system based on harmonic diffraction and free-form surface. By selecting the initial structure through the analysis of quantitative indicators and using Zemax macro language to control the process of system optimization, a dual-band (3~5 μm, 8~12 μm) infrared imaging optical system with a focal length of 1 200 mm and a diameter of 300 mm was designed. When the system is at a spatial frequency of 10 lp/mm, the value of the modulation transfer function in the 3~5 μm band is greater than 0.6, and the value of the modulation transfer function in the 8~12 μm band is greater than 0.45. The root-mean-square radius of the diffuse spot of the system's dual-band full field of view is less than 25 μm. The system realizes non-thermal sensitization in the working temperature range of -60℃?~?+60℃. The designed system not only has a simple structure, meets the requirements of various technical indicators, but also has high realizability, which is of great significance to the further development of the airborne dual-band infrared imaging optical system.