Based on the development of IC industry, in order to meet medium CMP to the high plainness, proper polishing speed and low ion pollution. Through many-times-growth craft, the silica sols with different sizes ranging from 58.1 nm to 106.2 nm are prepared by low-cost water glasses in the given condition. The purification craft of a three steps resin exchanging has solved the problem that the content of ions in the silica sol prepared through the original craft is high. Have applied the whole set of production technology to enlarged experiments and achieved better actual results. Finally the silica sol prepared behaves well in the course of medium CMP.