A set of dual-wavelength infrared optical system was designed for airborne photoelectric detection equipment, based on dual-wavelength 320×256 element cooled thermal IR focal plane arrays (FPA) detector.The system adopts the combination of three kinds of material of Ge, ZnSe and ZnS to achieve athermalization design.By introducing harmonic diffractive surface and aspheric surface, the chromatic aberration and the off-axis aberration are well corrected, and the system structure is simplified.The system, working in the wave band of 3.7~4.8 μm /7.7~9.5 μm and with the F/number of 2, is consisted of 6 elements, and has 100% cold shield efficiency.The image quality evaluating results show that: the performance of the dual-wavelength infrared optical system is very well in at temperature from -60 ℃ to +70 ℃.