To solve the problem of single point diamond turning(SPDT), technology was used for the processing of large diameter and chord height aspheric aluminum mirrors, which have the problems of limited lathe guide stroke, limited rotary volume of worktable, and low processing quality. To process a concave aspheric aluminum mirror with a diameter of ?682 mm and a chord height of 220 mm, first, a three-axis linkage processing method based on SPDT was proposed, which adds rotary b-axis on the basis of two-axis processing, such that the guide rail travel and table rotation volume can meet the processing requirements. Then, the special cage fixture was designed, and the influence of the number of supporting rods, the diameter of supporting rods, and the thickness of upper and lower connecting plates on the jig-work piece deformation characteristics were examined using the finite element method. The influence of different factors on the maximum deformation of jig and work piece was evaluated via range and variance analysis. A set of optimal jig design parameters was obtained, that is, the number of jig support rods was 24, the diameter of the rods was 22 mm, and the thickness of the upper and lower connecting plates was 25 mm. Finally, the aluminum mirror was fixed on the optimized cage clamp, and the processing of the ?682mm aspheric aluminum mirror was realized through three-axis linkage processing. The test results show that the surface accuracy Pv of the tool adjusting part was 0.6 ?m, and the surface roughness Ra was approximately 10.1 nm. It can be considered that the surface accuracy and surface roughness of the ?682 mm aspheric aluminum mirror can meet the requirements. This study can provide a theoretical basis and technological reference for the processing of the same type of large aperture and high chord aspheric mirror.