Polysilicate aluminum ferric sulfate(PSAFS) is presented as one kind of new polymerized inorganic coagulants. The advantage of aluminum and ferric homopolymer are still contained while the disadvantage is thrown away such as high Al residuals of polyaluminum chloride(PAC) in treated water and the poor stability of polyferric chloride(PFC).Because of those,considerable interest and attention have been paid to PSAFS home and abroad in recent years. PSAFS is prepared from aluminum sulfate and ferric sulfate and water glasses. The influence of different factors(the stability of ploy-silicic acid,Al/Fe/Si molar ratio,etc.) in flocculation is studied and the optimal compounding condition is obtained finally.