Because a harmonic diffractive optical element has special achromatism and athermalization, it can be applied to infrared dual-band search and tracking optical system. In this paper, we make the device that can be used in the wavelength ranges 3.7—4.8 μm and 8.7—11.3 μm; It has an efficient focal length 200 mm, the F/number 2.5, the full field of view is 5°. The environment temperature can be changed between -80 ℃—200 ℃. The practical design shows that the system not only shows good image quality in mid-and long-wave, but also the advantages of compact structure, high rates of transmission and athermalization.