On the basis of permanent magnet Magnetorheological Finishing (MRF) machine, a magnetorheological finishing method by combining a 4-axis machine with a variable removal function was developed to figure the sphere surface in a raster path. The principle of equal area observed on the raster path and the dwell time algorithm based on matrix multiplication computation were discussed. A mechanical compensating strategy based on the 4-axis machine was analyzed, including machine compensating and algorithm compensating of the variable removal function. By using water based magnetorheological fluid with the polishing powder of cerium oxide, a polishing experiment on a convex made of BK7 material with a diameter of 80 mm and curvature radius of 800 mm was carried out. The experiments show that error values( PV and RMS) have been converged from 117.47nm and 22.78 nm to 60.80 nm and 6.28 nm respectively with just one polishing cycle (5.5 min). The result demonstrates that strategy of variable removal function compensating based on the 4-axis machine can effectively figure the sphere and asphere surfaces determinately by the MRF, which reduces the cost of the MRF machine, and pushes its applications to the the usual optical shop.