this invention concerns the preparation of certain polyester , which includes amino acid in backbone , and active drug in side chain . it has the general formula as follows : wherein r 1 is aspartic acid , serine , glutamic acid or lysine ; r 2 is active drug with reactive group . the preparation of this polymer has three steps : firstly the polymerization of lactide or lactone with derivative of morpholine - 2 , 5 - dion , secondly the deprotection , finally the bonding with drug . through introducing amino acid comprising reactive group to biodegraded polyester , the antigen of the degraded result can be avoided , and it can be absolutely bioabsorbed . because polymer includes carboxyl , hydroxy and amido , it can have effect directly , or after being degraded . this invention can be used as the coating of medical instruments , and can be prepared for the implant or other regent .