The invention relates to a method for preparing polymeric ferric aluminum silicate sulfate by using waste sulfuric acid generated in the production process of pyrite residue, coal ash and polyvinyl alcohol. The method comprises the following steps: firstly carrying out ball-milling mechanical activation on a mixture of pyrite residue and coal ash, and then leaching iron and a part of aluminum in the mixture by using the waste sulfuric acid to obtain an acidic leaching solution; carrying out a reaction between remaining residue and sodium hydroxide to convert silicon and aluminum in the mixtureinto soluble sodium silicate and sodium metaaluminate, and thus preparing an alkali conversion solution; then adding the alkali conversion solution into the acidic leaching solution, and carrying outhydrolysis and polymerization on iron, aluminum and silicate in the mixture by adjusting the pH value of the solution to prepare a polymeric ferric aluminum silicate sulfate composite coagulator. According to the method, the useful ingredients inside the waste sulfuric acid generated in the production process of pyrite residue, coal ash and polyvinyl alcohol are sufficiently utilized, the composite coagulator with the advantages of iron, aluminum and silicic acid is prepared, and efficient and high value-added resource utilization of the waste sulfuric acid generated in the production processof pyrite residue, coal ash and polyvinyl alcohol is realized.