The invention discloses a method for preparing polyester by ring opening polymerization and belongs to the technical field of high polymer material synthesis. According to the method, cyclic ester is used as a reactive monomer, metformin is used as a catalyst, and organic alcohol is used as an initiator; the above ingredients are mixed and stirred to carry out a bulk polymerization reaction; and separation is finally carried out to obtain polyester. The method for preparing polyester has characteristics of simple process, low cost and efficient reaction rate. The method of the invention has advantages of fast reaction, controllable process, mild reaction condition, low catalyst toxicity and narrow molecular weight distribution.