Method for preparing polyhydric alcohol through waste polyester bottles

The invention relates to a method for preparing polyester polyol through waste polyester bottles. The method includes the steps that after the waste polyester bottles are subjected to alcoholysis, flame-retardant phthalic anhydride is introduced to be esterified with alcoholysate; after small molecular alcohol and water are removed, polyether-tribasic alcohol with the PH value ranging from 8 to 11 is added to be further subjected to esterification and condensation, the acid value of the obtained polyester polyol is adjusted, and thus the polyester polyol which is low in cost and excellent in flame retardant property and physical and mechanical property is prepared. The synthetic product can be used for producing polyurethane foam plates for building heat insulation.