In order to verify the machining capability of ultra-gradient mirror figuring by ion beam, the figuring of ultra-gradient mirror by 5-axis ion beam is investigated. First, based on the existence measure condition, the experiment scheme is presented and the surface figure date is processed. The dwell time of the area exceed the machine range is compensated. Then the post processing for 5-axis ion beam figuring machine is established, the numerical control generate software is programed based on the post processing, and the correctness of the numerical control program is verified through experiment. Finally, the off-axis figuring experiment of ultra-gradient mirror by 5-axis ion beam is carried out, and through five iterations, the surface figure is improved from initial condition: the peak valley (PV) value is 57.983 nm, the root mean square (RMS) value is 9.406 nm to final condition: PV value is 11.616 nm, RMS value is 1.306 nm. Convergence ratio reaches 7.20. The results indicate that ultra-gradient sphere mirror can be figured by 5-axis ion beam figuring machine to achieve high efficiency and precision, and the figuring of off-axis mirror by ion beam figuring is verified at the same time.