Method for efficiently preparing and purifying lactide

The invention provides a method for efficiently preparing and purifying lactide. The method comprises the steps of polymerization, depolymerization and purification. The polymerization comprises prepolymerization, primary polymerization and secondary polymerization; the depolymerization is carried out in a film evaporator; the depolymerization process is carried out in the film evaporator, and a specific process is controlled, so that the retention time of the lactic acid oligomer in the reactor is relatively short, the depolymerization time is shortened, the carbonization of materials and the generation of M-lactide in the depolymerization process are avoided, meanwhile, the depolymerization temperature can be reduced, the depolymerization temperature is 170-190 DEG C, preferably 175-185 DEG C, and the depolymerization efficiency is greatly improved. The depolymerization time is 0.5 to 1 hour. According to the method, the free acid content, the moisture content and the racemization degree of the crude lactid