The present invention relates to a polyester resin composition capable of having an excellent shrinkage rate, being heat-shrunk at a low temperature, and improving adhesion properties, without impairing insulation properties in a state of heat-shrinkable films, and a method for preparing the same. The present polyester resin composition comprises: a copolymerized polyester resin including a dicarboxylic acid-derived residue including a residue derived from an aromatic dicarboxylic acid; and a diol-derived residue including a residue derived from 4-(hydroxymethyl)cyclohexyl methyl 4′-(hydroxymethyl)cyclohexane carboxylate, and a residue derived from 4,4-(oxybis(methylene)bis) cyclohexane methanol, an alkali metal compound, and an alkaline earth metal compound, wherein the alkali metal compound and the alkaline earth metal compound are contained in an amount that the content ratio of the alkali metal element/the alkaline earth metal element derived therefrom is 0.01 to 1.