A method for synthesizing polylactide (PLA). The method employs RCOOCG, a nontoxic carboxylate of cyclic guanidine (CG), as a catalyst and non-toxic organic alcohol R′OH as an initiator to catalyze/initiate a living ring-opening polymerization of lactide (LA). The method includes: 1) adding monomer LA, the catalyst RCOOCG, and the initiator R′OH to a reaction still, where an initial molar ratio of the monomer to the catalyst is [LA]0/[RCOOCG]0=1000-1500:1, an initial molar amount [R′OH]0 of the initiator R′OH is determined by a number average molecular weight (Mn) of a target product PLA; 2) removing air in the reaction still, sealing the reaction still after a pressure in the reaction still is constant at between 1.0 and 0.1 torr; stirring and heating the reaction still in 30 to 40 minutes to a temperature between 95 and 96° C.; and allowing reactants in the reaction still to polymerization.