Terahertz wave is an electromagnetic radiation area with significant scientific value. This paper has integrated 14-50m terahertz wave into multi-band imaging system

based on the special large dispersion capability of harmonic diffractive element. The longitudinal aberration is simultaneously limited less than 0.75mm in the five harmonic wave bands (15.8-16.2m

18.5-20m

23-25m

30.5-33.5m and 46-50m). In each harmonic wave band

the magnification changes as a function of wavelength

which creates image registration error. To compensate for this shortcoming

the zoom optical system is designed by means of optical two- component. The problem is solved successfully in the theory. The design results show that the image height keeps 6.74mm in every harmonic wave band and the zoom optical system still realizes aberration compensation action. In the five bands

each optical transfer function approaches the diffraction limit in 10 cycle/mm. Finally

the system realizes the requirement of portability

minitype and ease fabrication.