The invention discloses an ultra-
wide field-of-view off-axis three-reflector 
optical imaging system which comprises a main reflector with positive focal power, a second reflector with negative focal power, and a third reflector with positive focal power. The reflecting surface of the main reflector is a spherical surface. The reflecting surfaces of the 
secondary mirror and the third reflector are flat spherical secondary curved surfaces. The curvature radii of top points of the three reflectors approximately satisfy a flat 
image field condition. The main reflector generates a 
real image of a far scene between the main reflector and the second reflector. The second reflector and the third reflector perform 
relay imaging of the 
real image on an 
image plane. An aperture diaphragm is placed on the front focal plane of the third reflector, thereby realizing an image-side telecentric 
optical path. The main reflector inclines along an X-axis relative to incident main light of a 
central field-of-view. The second reflector is coaxial with the third reflector. After the main light of the 
central field-of-view in a 
meridian plane is reflected by the main reflector, an included angle is formed between the reflected main light and the common 
symmetric axis of the second reflector and the third reflector. The ultra-
wide field-of-view off-axis three-reflector 
optical imaging system has advantages of ultra-
wide field-of-view, intermediate relative aperture, wide working 
wave band, no image-side telecentric barrier, small object-side 
distortion, high 
imaging quality, etc. The ultra-wide field-of-view off-axis three-reflector 
optical imaging system is suitable for earth imaging observation field of a spatial camera, an 
imaging spectrometer, etc.