Abstract: Aiming at the design requirement of miniature reflective optical system, the new off-axis three-mirror optic path and freeform surface were adopted, the compact freeform three-mirror was realized with 1.8 F^# and 7.5° diagonal field of view(FOV).The light overlaps multiple times in the system, and the full FOV average of the system wave aberration is 0.017 λ (λ= 10 μm). Compared with the aspherical off-axis three-mirror system, the system has the advantages of small size, high transmittance, large FOV, no central obscuration, and multi-band imaging, etc.