The invention relates to the technical field of organic polymer polymerization reaction, in particular to a method for preparing polyglycolic acid through glycolic acid polymerization. The method comprises the steps that condensation polymerization is performed after glycolic acid purification and dehydration, then a polymerization reaction additive is added, thermal polymerization is performed in a vacuum airtight container, microwave polymerization is performed, and filtering, washing and drying are performed at room temperature after cooling to obtain a polymer. The method overcomes the shortcomings of complicated operation, high production cost, difficult processing additive dispersion of an existing polyglycolic acid preparation process. In addition, the prepared polyglycolic acid is excellent in performance, low in water content and high in molecular weight, the content of residues such as a monomer, a solvent, a catalyst, heavy metals and the like is low, the biocompatibility is good, and the requirements of multiple grades of raw materials such as food-grade and medical-grade raw materials can be met.