The invention relates to a method for preparing high- performance water purification agent. The activated sodium silicate is polymerized with ferric sulfate, and modifies with chloride ion during process of producing polymeric ferric sulfate. It comprises following steps: adding 5- 6 units sodium silicate ((M=3.3,SiO2%=26) into autoclave, adding 20- 13 units water, stirring evenly, adding 2 units industrial concentrated sulfuric acid quickly, reacting at 40- 50 Deg. C for 0.5 hour, then adding 42- 45uints 90% ferric persulfate, 1- 2 units sodium chloride, 3- 5 units aluminum sulphate acetate, 4- 5 units industrial concentrated sulfuric acid, dropping 13- 14 units 27.5% hydrogen dioxide solution slowly, controlling temperature to be lower than 60 Deg. C, reacting for 2 hours, dissolving 1- 2 units sodium orthophosphate in 10- 7 units water and adding them into autoclave, reacting for another 0.5 hour, curing 24 hours and getting final product. The product is characterized by apparent functions of removing color, odor, oil, disinfection and removing COD and BOD.