The invention provides a method for preparing aluminum polychlorid from byproducts generated in methyl diethyl phosphate production. The method comprises the following steps: (1) dispersing byproductsolid dregs into a solvent, analyzing, and collecting a solution; (2) cooling the solution with water, and separating aluminum chloride hydrate crystal; (3) hydrolyzing and polymerizing the aluminum chloride hydrate crystal, and aging, thereby obtaining the aluminum polychlorid. By adopting the method, solid dregs rich in sodium tetrachloroaluminate are adopted as a raw material, an inorganic polymeric flocculant, namely the aluminum polychlorid, is successfully prepared through steps of simple processes, and not only is the problem of waste disposal solved, but also a high additional value product can be successfully made.