Abstract: To meet the requirements of infrared dual-band imaging system performance testing and evaluation applications, an off-axis three-mirror system operating at 3m ~ 5m and 8m ~ 12m band was designed for infrared dual-band scene simulator. The aberration characteristics of off-axis three-mirror system with aperture departing far from the primary mirror were analyzed based on the coaxial three-mirror imaging theory, the design methods of the optical structure and aberration balance of the off-axis three-mirror with large eye relief and large relative aperture were discussed. The focal length of the system is 255 mm with F# 3, the field of view is 6 4.5 , the eye relief is 750 mm, the image quality of the system approaches the diffraction limit with MTF0.65 for long-wave infrared band and MTF 0.4 for mid-wave infrared band at 10lp/mm spatial frequency. The system is characterized by large field of view, large eye relief, high resolution and compact structure.