In this paper, with sodium carbonate as an auxiliary agent in the fly ash, the mixtures were sintered at 880°C for 90 min, then dissolved by different concentrations of sulfuric acid respectively for extracting the hydrated silica. The results show that when the activated ash fly is dissolved by different concentrations of sulfuric acid, the dissolution rates of ash fly increase with the concentration of sulfuric acid increasing. Impurities of aluminum and iron atoms are present in an orderly array of micro-andalusite structure in the generative hydrated silica. When they are dissolved by sulfuric acid for 2%(wt%), the orderly structure is destroyed obviously, and the aluminum and iron atoms are separated out. The purity of hydrated silica can reach 99.9%.