The invention discloses a method for preparing regenerative cationic-dye dyeable polyester with a waste-polyester alcoholysis method. The method includes the steps of 1), pretreatment of waste polyester; 2), alcoholysis of the waste polyester; 3), quality regulation of esters and adding of isophthalic acid-5-sodium and polyethylene glycol as a comonomer; 4), polycondensation and slicing preparation, wherein the cationic-dye dyeable polyester finally obtained is 0.55-0.65dl/g in intrinsic viscosity, 220-230DEG C in melting point, 5.5+/-0.3% in content of diglycol and smaller than 8 in chroma value of b. The obtained regenerative cationic-dye dyeable polyester is applicable to preparation of cationic-ion dyeable poester filaments and short fiber, and high-value recycling of the waste polyester is realized.