The topological structure and coordinate transformation of Magnetorheological Finishing (MRF) machine tool KDMRF-1000 were analyzed and its kinematics was calculated,then the post processing algorithm model of MRF for an optical lens was established. According to the characteristic of four-axis linkage machine tool, the model was treatmented approximately. By taking the post processing of a spherical lens as an example, a post processing algorithm that can be used for a raster tool path was deduced and the errors imported by this approximate treatment were analyzed. By simulating the effect of the algorithm on the spherical lenses with different apertures and relative apertures, the polishing range of this algorithm was achieved. Furthermore, a spherical part (K9, 200 mm in diameter, f=640 mm) was polished by MRF, experimental results show that obtained errors are less than 65 nm PV and 9 nm RMS in excluding the edge effect. This experiment proves the veracity of post processing algorithm and the feasibility of four-axis linkage approximate treatment. The work done in this paper has an important purpose for polishing the optical lenses with different apertures and shapes.