Considering the requirements of aberration and athermalized aberration, the concept of double harmonic diffractive element(HDE) was put forward based on the HDE. An infrared athermalized optical system that consists of two kinds of infrared materials composed AMTIRI and Ge was designed with work dualband of 3~5 μm and 8~12 μm and the large relative aperture of 1.2. This system had stable performance in the range of (－40~+60)℃, was able to be applied in the dualband staring focal plane array detector with the pixel size of 25 μm and the number of pixel 640×480, and could distinguish the goals in 3 m when the role of distance was 2 km. It retained the unique properties of common diffractive lens, and could obtain the same focal power in a series of separate wavelength. So, it can greatly improve the diffraction efficiency of each band.