The invention discloses a method for preparing polyester through ring-opening polymerization, and belongs to the technical field of polymer material synthesis. The method comprises the following steps: carrying out a bulk polymerization reaction on cyclic ester as a reaction monomer by using bis(trifluoromethylsulfonyl)imide as a catalyst and organic alcohol ROH as an initiator, and separating obtained products to obtain the polyester. The method for preparing the polyester, provided by the invention, has the advantages of simplicity, low cost, high reaction rate, fast reaction, controllable process, mild reaction conditions and narrow molecular weight distribution.