A method of preparing a biodegradable polyester-based polymer including: a prepolymerization step of polymerizing a dicarboxylic acid compound, a diol compound, and a branching agent, which is represented by Formula 1 below, at 160-220°C to produce a prepolymer; and a condensation polymerization step of performing a condensation polymerization of the prepolymer at 200-250 °C, under a vacuum pressure of 13.33 to 266.65 Pa (0.1 to 2 Torr): wherein X is selected from the group consisting of a substituted or unsubstituted C 6 -C 30 arylene group and a substituted or unsubstituted C 2 -C 30 heteroarylene group, X may include at least one -COOR', wherein R' is selected from the group consisting of a hydrogen atom, a halogen atom, a substituted or unsubstituted C 1 -C 20 alkyl group, a substituted or unsubstituted C 2 -C 20 alkenyl group, a substituted or unsubstituted C 2 -C 20 alkynyl group, a substituted or unsubstituted C 6 -C 30 aryl group, a substituted or unsubstituted C 7 -C 30 arylalkyl group, a substituted or unsubstituted C 2 -C 30 heteroaryl group, and a substituted or unsubstituted C 3 -C 30 heteroaryl akyl group, Y is selected from the group consisting of a substituted or unsubstituted C 1 -C 30 alkylene group, a substituted or unsubstituted C 1 -C 30 heteroalkylene group, a substituted or unsubstituted C 3 -C 30 cycloalkylene group, a substituted or unsubstituted C 3 -C 30 heterocylcoalkylene group, a substituted or unsubstituted C 6 -C 30 arylene group, and a substituted or unsubstituted C 2 -C 30 heteroarylene group, and n is 1 to 100. a prepolymerization step of polymerizing a dicarboxylic acid compound, a diol compound, and a branching agent, which is represented by Formula 1 below, at 160-220°C to produce a prepolymer; and a condensation polymerization step of performing a condensation polymerization of the prepolymer at 200-250 °C, under a vacuum pressure of 13.33 to 266.65 Pa (0.1 to 2 Torr): wherein X is selected from the group consisting of a substituted or unsubstituted C 6 -C 30 arylene group and a substituted or unsubstituted C 2 -C 30 heteroarylene group, X may include at least one -COOR', wherein R' is selected from the group consisting of a hydrogen atom, a halogen atom, a substituted or unsubstituted C 1 -C 20 alkyl group, a substituted or unsubstituted C 2 -C 20 alkenyl group, a substituted or unsubstituted C 2 -C 20 alkynyl group, a substituted or unsubstituted C 6 -C 30 aryl group, a substituted or unsubstituted C 7 -C 30 arylalkyl group, a substituted or unsubstituted C 2 -C 30 heteroaryl group, and a substituted or unsubstituted C 3 -C 30 heteroaryl akyl group, Y is selected from the group consisting of a substituted or unsubstituted C 1 -C 30 alkylene group, a substituted or unsubstituted C 1 -C 30 heteroalkylene group, a substituted or unsubstituted C 3 -C 30 cycloalkylene group, a substituted or unsubstituted C 3 -C 30 heterocylcoalkylene group, a substituted or unsubstituted C 6 -C 30 arylene group, and a substituted or unsubstituted C 2 -C 30 heteroarylene group, and n is 1 to 100. wherein X is selected from the group consisting of a substituted or unsubstituted C 6 -C 30 arylene group and a substituted or unsubstituted C 2 -C 30 heteroarylene group, X may include at least one -COOR', wherein R' is selected from the group consisting of a hydrogen atom, a halogen atom, a substituted or unsubstituted C 1 -C 20 alkyl group, a substituted or unsubstituted C 2 -C 20 alkenyl group, a substituted or unsubstituted C 2 -C 20 alkynyl group, a substituted or unsubstituted C 6 -C 30 aryl group, a substituted or unsubstituted C 7 -C 30 arylalkyl group, a substituted or unsubstituted C 2 -C 30 heteroaryl group, and a substituted or unsubstituted C 3 -C 30 heteroaryl akyl group, Y is selected from the group consisting of a substituted or unsubstituted C 1 -C 30 alkylene group, a substituted or unsubstituted C 1 -C 30 heteroalkylene group, a substituted or unsubstituted C 3 -C 30 cycloalkylene group, a substituted or unsubstituted C 3 -C 30 heterocylcoalkylene group, a substituted or unsubstituted C 6 -C 30 arylene group, and a substituted or unsubstituted C 2 -C 30 heteroarylene group, and n is 1 to 100.