In order to get enough information,and to avoid the big chromatic aberration of wide spectrum and the complexity of optical system,a harmonic diffractive/refractive optical system was established by making the best use of science value of terahertz.A 14～50 μm terahertz wave was integrated into multi-band imaging system based on the special large dispersion capability of harmonic diffractive element,and the longitudinal aberration was simultaneously limited to less than 0.75 mm in the five harmonic wave bands (15.8～16.2 μm,18.5～20 μm,23～25 μm,30.5～33.5 μm and 46～50 μm).In each harmonic wave band,the magnification changed as a function of wavelength,which created image registration error.To compensate this shortcoming,a zoom optical system was designed by binary optical component.The design results show that the image height keeps 6.74 mm in every harmonic band and the zoom optical system can realize aberration compensation action.In the five bands,each optical transfer function approaches to the diffraction limit in 10 lp/mm.These results indicate that the system can meet the requirements for portability,minitype and easy fabrication.