The invention provides a tri-block copolyester and a preparation method thereof. The method comprises the following steps of: reacting an aliphatic-aromatic copolyester with lactide in the presence of an organo-tin compound on an esterification condition in an inert atmosphere, wherein the aliphatic-aromatic copolyester comprises structure units shown in the formulas 2 and 3 in the specification,in the formulas 2 and 3, Ar is a group containing benzene ring, double benzene nucleus or anthracene nucleus, m is an integer between 1 and 20, n is an integer between 1 and 16, t is an integer between 1and 20, the molar ratio of the structure units shown in the formulas 2 and 3 is 1: (0.1-10), and the weight-average molecular weight of the aliphatic-aromatic copolyester is 6,000-600,000. The tri-block copolyester prepared by the method disclosed by the invention has excellent biodegradation performance, toughness and strength; moreover, products generated after the tri-block copolyester is biodegraded are non-toxic.