Magnesium Fluoride crystal processed under high temperature and high pressure, one of the infrared optic material, has been used in many areas. Magnetorheological finishing (MRF) is becoming an ultra-precision finshing technology because of its excellencies of high polishing efficiency, no wear and certain polishing. In this paper, at the first, the polishing characteristics are obtained by using traditional polishing method; and then, a new magnetorheological fluid suitable to finishing Magnesium Fluoride is developed. Compared to traditional polishing method, the method proposed here can yield higher surface quality and higher polishing efficiency.