The present invention relates to a method for preparing poly (lactic acid)-based copolymers. In the present invention, lactic acid prepolymers are synthesized via direct melt polycondensation, and then the lactic acid prepolymers are copolymerized with aliphatic polyesters diols and poly (lactic acid)-based copolymers with low molecular weight is obtained, and at last the poly (lactic acid)-based multi-block copolymers with high molecular weight, great biodegradation and definite ductility is produced via chain extension process. The method of the present invention is simple and feasible for industrial production and the copolymer resin product obtained is biodegradable and widely applicable.
