In this paper, the hydroxyl groups on the surface of graphene oxide (GO) were used to initiate the ring-opening polymerization of a lactic acid O-carboxyanhydride. GO grafted with poly (l-lactic acid) molecular chains (GO-g-PLLA) was prepared. Lactic acid O-carboxyanhydride has a higher polymerization activity under mild polymerization conditions. Thus, the functionalization of the polymer chains and obtaining poly (lactic acid) (PLLA) was easily achieved by ring-opening polymerization with 4-dimethylaminopyridine (DMAP) as the catalyst. The results showed that with this method, PLLA can be rapidly grafted to the surface of GO in one step. As a result, the chemical structure of the GO surface was altered, improving its dispersion in organic solvents and in a PLLA matrix, as well as its bonding strength with the PLLA interface. We then prepared GO/PLLA and PLLA/GO-g-PLLA composite materials and investigated the differences in their interfacial properties and mechanical properties. GO-g-PLLA exhibited excellent dispersion in the PLLA matrix and formed excellent interfacial bonds with PLLA through mechanical interlocking, demonstrating a significant enhancement effect compared to PLLA. The water vapor and oxygen permeabilities of the GO-g-PLLA/PLLA composite decreased by 19% and 29%, respectively.