Abstract: A method for preparing a thermal adhesive copolymerized polyester resin is disclosed. The method includes: a transesterification reaction in a reaction mixture of a diol compound according to Formula 1, for example, 2-Methyl-1,3-propanediol, an isophthalic acid (IPA) or an IPA derivative as an acid component, and polyethylene terephthalate; and a condensation polymerization reaction of the reaction mixture to prepare a copolymerized polyester resin. The copolymerized polyester resin has an intrinsic viscosity (I.V.) of 0.5 dL/g to 0.75 dL/g, a softening point (Ts) of 100° C. to 160° C., and a glass transition temperature (Tg) of 60° C. to 70° C. The copolymerized polyester resin can be effectively used in fields of clothing, automobile parts, as a thermal adhesive fiber. wherein R1 is an alkyl group having 1 to 4 carbon atoms, a and b are each independently an integer ranging from 0 to 3, and each solid line represents a single bond when a and b is 0.