Based on the properties of infrared radiation windows and harmonic diffractive element, harmonic diffractive element is successfully introduced to the system of infrared dual color detector. The rectification of the longitudinal aberration in two bands is simultaneously accomplished. Wavefront aberration is less than 1/4 wavelength and the modulation transfer function of dual band approaches or attains the diffraction limit. The results show that the properties of action spectrum of harmonic diffractive element are between refractive element and diffractive element. The system debases the demands to technical level. The practical design shows that the system not only has the advantages of compacted structure, few elements and high rates of transmission, but also has better weaken ray aberrations character. It can be used to put forward a new element for infrared optical designs.