The invention provides a method for synthesizing a polylactic acid which has no metallic residue and has controllable molecular weight. The method is as follows: a reaction solvent is added into lactide to prepare a lactide solution the mol concentration of which is between 0.5 and 1.5 M under the protection of inert gas, and an initiator and catalyst are added into the lactide solution in turn, and subjected to polymerization for 0.25 to 2 hours, wherein the mol ratio of the lactide to the initiator to the catalyst is 6-350 to 1 to 0.06-0.35; a terminator is added into the lactide solution, and subjected to termination reaction for 10 to 20 minutes, wherein the mol ratio of the initiator to the terminator is 1 to 1-5; and methanol is prepared, wherein the volume ratio of the reaction solution to the methanol is 1 to 10-20, the reaction solution is dripped into the methanol to form polylactic acid deposit, and the polylactic acid deposit is separated out, filtered, subjected to methanol washing for 2 to 3 times, and subjected to vacuum drying for 24 to 48 hours at a temperature of between 20 and 40 DEG C to obtain the product. The synthesis method can be performed under the condition of room temperature and normal pressure, has high product yield and simple technique, does not require the conditions of heating and decompression, is easy to operate, and can effectively control the molecular weight of the polylactic acid product; and the product has no metallic residue and can be used as a medical material.