The paper presents a newly developed optical polishing method, Ultrasonic-magnetorheological compound finishing (UMC finishing). The mechanism of UMC finishing is introduced in the paper. Experiments are carried out to study the surface quality of the optical glass K9 in UMC finishing. The result shows that the surface roughness Ra of the optical glass is 4.0nm, which is measured by the profile meter. The rules of surface roughness for optical glass by the main processing parameters are also acquired by the experiments. The study will be the basis for the further study of UMC finishing technology.