With the development of infrared technology, the demand for large aperture space infrared optics is increasing. Their manufacturing indexes are approaching the manufacturing requirements of visible optics gradually, which poses a higher challenge to the processing and testing technology of new infrared optics. A processing scheme of ultrasonic milling, robot griding and ion beam polishing was proposed for the manufacturing of large diameter ultra-thin silicon-based infrared lens with high gradient. This method overcame the defects of low efficiency and high frequency surface error caused by single process. In order to avoid the testing error caused by supporting force in contour testing of the aspherical convex side, flexible buffer supporting and three-point forced displacement supporting were applied in rough and fine polishing phases respectively, which effectively solved the supporting deformation problem in the testing of large aperture and high gradient ultra-thin lens. Through theoretical simulation and experimental verification, it was proved that the testing method had good consistency. An improved contour detection method was adopted to achieve accurate separation of supporting error in contour testing, and has effectively improved the limit accuracy of manufacturing. Finally, the machining accuracy of convex aspheric surface with large diameter mm infrared lens reached RMSλ/50 (λ=632.8 nm), which satisfied the design requirements.