In order to improve the manufacturing accuracy and efficiency of the secondary mirror in the off-axis three mirror anastigmatic (TMA), the combined fabrication and testing technologies of the off-axis convex aspheric mirror were studied. Firstly‚ the optical parameters, technical specifications and overall processing route of square (298 mm×264 mm) high order off-axis convex aspheric mirror were introduced. Then, the grinding processing strategy and the combined processing technology based on bonnet and pitch polishing were presented. At last, offner-type compensator was used to meet the back transmission null lens testing in the polishing stage, and then the surface distortion was corrected by ray tracing method, the final RMS value was 0.025λ (λ=632.8 nm), which meet the target requirements. The above combined processing technology and back transmission null lens testing detection scheme can significantly improve the processing accuracy and efficiency of off-axis convex aspheric mirror.