The present invention provides a direct polycondensation method for medical biodegradable polylactic acid (PLA) using creatinine catalyzed lactic acid. The invention uses commercialized creatinine (a type of biomaterial organic guanidine compounds—the arginine metabolite creatinine (CR) in human body) as the catalyst and industrial lactic acid (LA, mass content 85-90%, aqueous solution) as the monomer to synthesize the high biosafety of medical PLA in terms of second polycondensation without solvent (identity). Instead of tin catalysts having cytotoxicity, the catalyst used in the invention has high biocompatibility and biosafety. The synthesized PLA does not contain any metal and other toxic components; therefore, it can be used as the carrier for targeting drugs and controlled release drugs. In addition, the green catalyst and green processing method (no solvent applied and no toxic products produced) are used in the invention to synthesize the green biodegradable PLA with high biosafety. The polymerization reaction is simple and the raw materials required are low in costs, thus it is easy for industrialization. Further, the molecular weight distribution for all synthesized products is narrow and the molecular weight is controllable within 1.5-3.0×104.