The invention discloses a method for preparing regenerated flame-retardant polyester through a waste polyester alcoholysis process. The method comprises the following steps: 1) pretreating waste polyester; 2) performing alcoholysis on waste polyester; 3) controlling the quality of a compound ester, and taking an ethylene glycol solution of 2-carboxyethylphosphinicacid as a comonomer to be added; 4) performing condensation polymerization and preparing chips, wherein the intrinsic viscosity of the finally obtained regenerated flame-retardant polyester is 0.60-0.70dl/g, the melting point is 230-240 DEG C, the content of diethylene glycol is 3.5+/-0.3%, the chroma value b is less than 8, and the limit oxygen index is 28-34%. The regenerated flame-retardant polyester prepared by the method disclosed by the invention is high in limit oxygen index and excellent in flame-retardant property and can be used for preparing flame-retardant polyester filaments and short fibers, and high-valued recycling of the waste polyester is realized.