Abstract: To meet the requirements of short-range and large-screen projection system in domestic and foreign markets, it is necessary to reduce the processing difficulty and cost of projection lens. In this case, an ultra-short focal projection lens was designed. The catadioptric structure was selected, the principle of the relationship between object and image was used, the mirror surface type parameters were calculated by combining the object field of view, and finally the mirror surface type was obtained. This method was convenient to correct the distortion of large field of view and counteract the astigmatism and field curvature of refraction lens group. The whole optical system consists of 15 spherical lenses and one piece of aspheric mirror, with a total length of 286 mm, projection distance of 500 mm, projection screen size of 254 cm (100 inch), and projection ratio of 0.22. The relative illumination is greater than 85%, the MTF is greater than 0.5 at 0.4 lp/mm, the horizontal TV distortion is less than 0.55%, and the vertical TV distortion is less than 0.65%. Many design indexes of the system are better than that of the existing products on the market, and its coaxial spherical surface design is easy to process and adjust, which can effectively reduce the production cost.