An aliphatic polyester polymer compositions and preparation method thereof. The invention provides a bioresorbable aliphatic polyester copolymer, comprising the reaction product of a first polyester, a second polyester, and a coupling agent. Specifically, the first polyester and second polyester have the same repeat units, but different weight average molecular weights. Methods for preparing the aliphatic polyester copolymer are provided, in which the first polyester reacts with the second polyester in the presence of the coupling agent, undergoing copolymerization.