The high-gradient concave optics polishing is a problem in optical manufacture. The optical belt deterministic polishing method is one of the efficient ways to solve such optics polishing problem. We introduced the theory and method of the optical belt polishing technique, studied the feasibility of figuring the optical surface with the optical belt polishing method. To demonstrate an optical belt polishing process, a 80 mm K9 glass plane mirror was processed on a 5axis precise computer-numerically-controlled (CNC) machine. Its surface figure accuracy root-mean-square (RMS) was improved from initial 0.10 9 (= 632.8 nm) to final 0.02 8 through three ion beam polishing (IBF) processes, and the mean iterative convergence ratio was 1.3. The result shows that the polishing method has rapid convergence rate as well as machining accuracy on surface figuring. This experiment proves that the optical belt polishing can figure this kind of optical glass, which lays the foundation for figuring high-gradient optics.