In order to improve the polishing efficiency of magnetorheological polishing and realize the high-efficiency, high-quality and ultra smooth planarization of photoelectric wafer, a magnetorheological variable gap dynamic pressure planarization method is proposed. In this paper, the changes of material removal rate and surface roughness of sapphire wafer surface polishing with processing time under different variable gap conditions are studied, and the dynamic pressure flattening mechanism of magnetorheological variable gap is deeply analyzed. The results show that the dynamic change of polishing pressure and the extrusion strengthening effect of MR fluid can be produced by applying axial low-frequency extrusion vibration to MR polishing fluid, and the polishing efficiency and polishing effect can be significantly improved. After 120 min of magnetorheological variable gap dynamic pressure flattening, the surface roughness of sapphire wafer decreased from Ra 7 nm to Ra 0.306 nm, and the material removal rate was 5.519 nm / min. Compared with constant gap magnetorheological polishing, the surface roughness decreased by 49% and the material removal rate increased by 55.1%; By changing the moving speed of variable clearance, the flow field characteristics can be controlled. Choosing the appropriate workpiece pressing speed and workpiece lifting speed is conducive to improve the polishing efficiency and surface quality.