An infrared athermal optical system with F number of 1.2 for dual-band of 3~5 μm and 8~12 μm is designed. In this system, the view of vision angle is 22°, total length is 78 mm. Germanium and AMTIR I (mix of Ge, As and Se) and double-layer harmonic diffraction element are used to simplify the three-lens structure, decrease weight, and improve the quality of the imaging. Across temperature range of -40 ℃~60 ℃, the system has a stable performance and applies to gaze-type dual-band focal plane array detector with pixel size of 25 μm, and pixel number of 640 pixel×480 pixel. The design results show that, the modulation transfer function (MTF) is greater than 0.5 at different temperatures when the Nyquist frequency of detector is 20 lp/mm. The imaging quality is good, and athermal design is realized. It can distinguish a target of 3 m when the distance is 2 km. So this system can meet the needs of military reconnaissance.