Based on the negative dispersion and heat dissipation characteristics of harmonic diffractive optical elements, a wide-band optical imaging system with operating band of 0.40-2.50 μm is designed in this paper. The mathematical model of the bandwidth integral average diffraction efficiency of the double-layer diffractive optical element is established. The optimum design wavelength of diffraction element is determined by Matlab software. BaF₂, AL₂O₃, AL₂O₃-E and ordinary optical glass (KZFSN5, SF57) are used to design the refractive and diffractive hybrid 5-piece optical structure, and the wide-band common optical path confocal plane integration is realized by rational distribution of optical focal degree. Experimental results show that, the system has an effective focal length of 100 mm, the field angle of view is 9.4°, and the F-number is 2.8. The system achieves no thermo in the range of -40-60 ℃. At the Nyquist frequency of 50 lp/mm, the modulation transfer function (MTF) of 0.40-0.78 μm is greater than 0.6, and the MTF of 0.78-2.50 μm is greater than 0.5. Compared with the traditional wide-band optical system based on refractive lens, the system has the advantages of simple structure, small size and imaging quality close to the diffraction limit.