By the analysis of chemical mechanical polishing flow field about substrate, it is found that the closer to the center of the substrate film, the polishing rate is more uniform; at the edge it isn’t; at the same time near the rotation axis of polishing pad, the polishing rate is little, while far away from the axis the rate is lager. It’s needed to choose appropriate ration on the speed of polish pad and the speed of substrate, in order to obtain a substrate with better flatness. The change on the speed of polishing pad affects polishing rate larger than the change of substrate’s. With PLM-2 precision polishing machine and S8030N slurry, when the rotation speed of polishing pad is 60rom, the substrate’s is 40rpm, sapphire substrate with complete lattice has been obtained. While BOW of substrate is less than 8 microns, TTV of substrate is less than 5 microns, substrate surface roughness is less than 0.5nm.