A dual-band IR optic system with bands of 4.4~5.4 μm and 7.8~8.8 μm is designed on the basis of the special properties of Harmonic Diffractive Element （HDE）.The HDE element characteristic is analyzed and the structural parameters of the system are calculated.The optimization using multi-configuration in the software has been simultaneously accomplished.The results show that the MTF values are all over 0.5 at the resolution of 20lp/ mm,which displays a good optical performance.The HDE offers a new component for optical designs.