The fabrication and testing of a Ф420 mm fused silica high-order aspheric lens were presented. Current fabricating technologies were optimized, and robot grinding, polishing, and ion beam figuring were respectively applied to the fabrication of lens. For testing an aspheric lens, a spherical interferometer was used to test the concave surface of spheric lens, and the result shows that the test accuracy is 0.011λ-rms after the error of interferometer standard lens reference surface was removed. Furthermore, null compensators based on reflective auto-collimating method were taken to test the high-order convex aspherical surface,and the surface figure errors are 0.013λ-rms. Finally, a standard spherical mirror was used to test the transmission wavefront of processed aspheric lens with self-aplanatic form,and the transmission wavefront errors of aspheric lens is 0.013λ-rms. The testing results indicate that the entire specifications meet the requirements of design. The processing methods proposed by the paper are suitable for the fabrication of larger aspheric lenses and other larger aperture aspherical elements.