The invention discloses a method for synthesizing polymeric aluminum chloride by utilizing powdered rock phosphate. The method comprises the following steps: utilizing aluminium hydroxide and concentrated hydrochloric acid; heating the aluminium chloride at certain temperature; and simultaneously adding liquid supernatant obtained through reaction of the powdered rock phosphate and the concentrated hydrochloric acid to polymerize, thus obtaining the polymeric aluminum chloride. The method disclosed by the invention is simple, and is simple and convenient in operation and safe in production, and has the advantages of wide pH practical range, low production cost and no secondary pollution. By utilizing the method, the obtained polymeric aluminum chloride has better efficacy compared with the common polymeric aluminum chloride, waste are treated by waste, waste is turned into value, the environmental pollution is reduced, and higher economic and social benefits can be obtained.