In order to obtain enough information about the target and make full use of medium- and long-wave infrared spectral data, this article describes a harmonic diffractive/refractive (HDE) optical imaging system. Taking advantage of its special dispersion capability, the application of HDE in the infrared dual-band provides hundreds of spectral images in infrared band, medium-wave infrared band of 3.7—4.8 μm and long-wave infrared band of 8—12 μm. The design results show that: at 18 lines/mm, the optical modulation transfer function is greater than 0.55 in medium-wave infrared band, at 13 lines/mm the optical modulation transfer is greater than 0.5 in long-wave infrared band, in the circle of 30 μm radius, the encircled energy is greater than 85% in medium-wave infrared band, in the circle of 38 μm radius, the encircle energy is great than 80% in long-wave infrared band, the spectral resolution is 0.039 μm at 4.25 μm and 0.072 μm at 8.5 μm respectively. So the optical modulation transfer function in infrared dual-band is close to the diffraction limit, and the encircled energy meets the energy requirement of one pixel of existing domestic detectors.