In view of the phenomena that the easy appearance of serious roughness and surface defects, the polishing mechanism and the influence of the main parameters on the polishing quality are researched. The research indicates: the superficial hydrolysis layer on the polishing surface has the vital significance to the formation of ultra smooth surface. The super big and hard grains of the polishing powder are the main reason of the surface defects. According to the variational law of the polishing powder's main parameters with polishing time, the concept of polishing cycle and the method of pretreatment on the polishing powder are proposed. viz. the polishing powder is pretreated on the initial stage period and the workpiece is polished on the intermediate stage. The experimental result indicates: this method can truly reduces surface roughness and the surface defect, and the polishing quality and efficiency are effectively enhanced.