A method of preparing a poly(ether ester) copolymer is provided, and more particularly, a method of preparing a poly(ether ester) copolymer having a high content of a polyether and high viscosity by using a dicarboxylic acid as a starting material is provided. According to the present invention, a dicarboxylic acid of which production cost is low is used as a starting material to prepare a poly(ether ester) copolymer having a high content of polyether and having a high viscosity without solid-phase polymerization.