The thermal behavior and structural of banana starch films with degraded PET is discussed (S/PET_deg). The PET degradation and the characteristics of A/PET_deg films were analyzed by Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD) and simultaneous thermal analysis (STA). Terephthalic acid, acid dimers and residual macromolecular chains were the degradation products of PET, which showed different thermal behavior. The thermal stability of the A/PET_deg composite films was lower than the individual components; this behavior was related to the breakdown of the starch amorphous domains (branching points of the amylopectine molecule) and due to the catalytic effect caused by the terephthalic acid during the thermal process. Additionally, the barrier to water absorption of the A/PET_deg films was increased depending on the content of PET_deg.