The invention relates to a method for preparing polyester by catalyzing cyclic anhydride/epoxide ring opening alternate copolymerization with potassium acetate. According to the method, potassium acetate catalyzes an epoxy compound and cyclic anhydride to be subjected to ring-opening alternate copolymerization to synthesize polyester, wherein potassium acetate is used as a catalyst to catalyze thecopolymerization of the cyclic anhydride and the epoxy compound, the reaction formula is defined in the specification, R1 and R2 are anhydride substituents, are the same or different, and simultaneously or independently are a hydrogen atom, a double bond, a benzene ring or a norbornene ring, and R3 and R4 are epoxy compound substituents, and are simultaneously or independently H, methyl, benzenering or cyclohexyl. The preparation method comprises the following steps: carrying out a polymerization reaction on excessive epoxide, cyclic anhydride and potassium acetate by using a normal-pressurebulk polymerization method, precipitating the polymer, filtering, and drying. Compared with the existing metal organic catalyst and non-metal organic catalyst, the potassium acetate provided by the invention has low biotoxicity, low price and simple preparation method, and has great application prospects.