The invention discloses a method for synthesizing polylactic acid through a mixed catalyst. The method comprises the steps that under the conditions of the mixed catalyst, 5-20 KPa, 70-120 DEG C and inert gas protection, lactic acid is subjected to a stirring reaction for 1-5 hours to obtain a low polymer of lactic acid, then the pressure is lowered to 0.2-1 KPa, the temperature is raised to 120-180 DEG C, and the stirring reaction is continued and lasts for 1-10 hours to obtain a lactic acid oligomer, wherein the mixed catalyst is prepared from a rare earth compound, a titanium type metal compound and a molecular sieve, and the mass ratio of the rare earth compound to the titanium type metal compound to the molecular sieve is 1 to (0.07-0.15) to (1-2); the lactic acid oligomer is obtainedthrough reaction under the conditions of 0.01-0.8 KPa, 180-230 DEG C and inert gas protection for 12-30 hours. According to the method, the utilized catalyst is higher in activity, the process is more convenient, and the prepared polylactic acid is high in performance, high in thermal stability and narrow in molecular weight distribution.