A multifunctional epoxide chain extender (ADR4370S) was used to increase the molecular weight of poly(trimethylene terephthalate) (PTT). And the effects of ADR4370S content on the molecular structure, melt viscosity, and rheological properties of PTT were studied. It is found that a star-type topological structure is formed in PTT by introduction of ADR4370S, and the balance torque, intrinsic viscosity, and molecular weight are increased by increasing ADR4370S dosage. The rheological measurement results show that the elastic modulus, complex viscosity, and shear thinning behavior of long-chain branching PTT are increased with the concentration of ADR4370S. The presence of broadened relaxation time spectrum and a long relaxation time mode for the PTT with 1.50 wt% ADR4370S demonstrate that the cross-linking reaction occurs, and the gel forms in the PTT system.