We proposed an infrared dual-band common aperture optical system, without thermalization, based on harmonic diffraction in this study. The optical system has two working bands of 3-5 .m and 8-12 .m, a focal length of 45 mm, a value of F/# of 2, and a resolution ratio of the frigorific double-color detector of 320×256 with a 30 .m pixel size. The harmonic diffractive optical element improves the large dispersion problem of the diffractive optical element over a wide wavelength band and solves the severe dispersion problem of the diffractive optical element over a wide wavelength band and low diffraction efficiency. The optical system uses harmonic diffractive optical elements to widen the chromatic aberration of the band and temperature range, so that the mid and long-wave infrared diffracted at different diffraction orders, to achieve resonant confocal imaging, and the dual-band infrared optical system are corrected using fewer optical components. This optical system also reflects the incomparable advantages of traditional optical systems with respect to improving image quality, reducing volumetric weight, and wide-band a thermalization. With the further investigation of the research and development (R&D) and manufacturing technology of dual-band detector harmonic diffractive lenses, this dual-band optical system could be widely used in military systems such as target tracking, identification, and precision strikes.