The invention relates to a no-blocking pure reflection optical system. The system is of an eccentric three-mirror structure and comprises a primary mirror, a secondary mirror, an aperture diaphragm, a ternary mirror and a detector focal plane which are arranged along the optical path from the object space to the image space in sequence, wherein the primary mirror, the secondary mirror and the ternary mirror are all reflecting mirrors, the primary mirror is a concave-paraboloid mirror, the secondary mirror is a convex hyperboloidal mirror, and the ternary mirror is a free-form-surface mirror; light is reflected on the surface of the primary mirror, the surface of the secondary mirror and the surface of the ternary mirror in sequence and blocking is avoided, and light reflected by the three mirrors irradiates on the focal plane perpendicularly. The system has the advantages of being wide in wave band, large in visual field, small in distortion, small in F number, free of blocking, telecentric in image space, free of chromatic aberration or thermal aberration and the like and is suitable for high-resolution observation.