The main feature of the direct method is that the synthesized PLA does not contain a catalyst. Therefore, when the polycondensation reaction is carried out to a certain extent, the system will have an equilibrium state, which requires heating and pressurizing to break the reaction equilibrium, and the reaction conditions are relatively harsh.In recent years, through technological innovation and improvement, direct polymerization method has made certain progress. It should be used in industrial large-scale production in the near future with the continuous maturity of technology.Lactic acid was heated slowly in inert gas and decompressed slowly to dehydrate and condense directly.