Abstract: With the rapid development of precision guidance technology, simultaneous detection of both bands has become very important, the demand of the corresponding optical system and its design increase rapidly. According to the harmonic diffraction element with the characteristic of achromatism, athermalization, same focus on each low diffractive level and high diffractive efficiency, using the CODEV optical design program, a coaxial dual-band IR optical system was designed that worked at-45~ +71℃ in the existing processing conditions of diamond cutting. The modulation transfer function(MTF) approached to the diffraction limit in the whole range of temperature and no need to focus, there was no vignetting in optical system and the efficiency of cold shield reached 100%. The dual band infrared optical system has the advantages of compact structure, fewer elements and high transmittance, it is a qualitative leap for improving detection accuracy and recognition ability of the seeker.