Infrared detection system, due to its high stability and all-weather adaptability, has been widely applied in civil and military areas. In this paper, based on the refractive/diffractive hybrid structure and the passive athermalization, a dualband infrared optical system with large relative aperture (F=1) is designed, which has excellent performance in the correction of thermal aberration, chromatic aberration and second spectrum between-40°C to 60°C. By precisely arranging the double-layer diffraction element, the system designed is simplified effectively, which contains only four lenses. Meanwhile, the optical layout has the advantages of lower weight and smaller volume. The MTF in mid-wave infrared is larger than 0.6, which demonstrates good capacity of target recognition and anti-inference, and thus it is suitable for practical usage in the field of aviation remote sensing. © 2019 SPIE.