Based on the imaging three-mirror aberration theory as the design basis, using a small field angle offset setting and a one-dimensional inclined plane mirror to fold the optical path in front of the main mirror, a coaxial large relative aperture imaging optical system with a high compression ratio was optimized. The focal length of the camera is 2.5 m, the image F is 6.3, the imaging field of view angle is 0.6°×0.3°, and the optical transmission function in the visible-near infrared band of 400-900 nm is greater than 0.41 at the spatial frequency of 91 lp/mm. It is better than 0.6 at 20 lp/mm, the imaging quality is close to the diffraction limit, and the imaging consistency is high over the full field of view. The length of the optical system has a high compression ratio greater than 1/5.6 times the system focal length and 1.1 times the diameter of the main mirror. The three mirrors are all conic without high-order aspheric coefficients and non-off-axis spatial layout. The tolerance analysis shows thatthe optical system is easy to realize by engineering and has a wide application prospect in the field of compact commercial imaging altimetry optical cameras with multi-star networking.