Disclosed is a method for preparing a polyester resin, wherein phosphate additives are used in preparing polyester resin to improve the reactive property of the esterification reaction or transesterification, as well as the flame retardancy property and the color stability of a polyester resin. The method for preparing a polyester resin comprises the steps of: esterification reacting and/or transesterifying diacid ingredients diol ingredients in the presence of one or more phosphate additives selected from the group consisting of the compounds as defined in Chemicals in the detailed description of the invention; and polycondensing the products obtained from the esterification reaction and/or transesterification.