Polybutylene adipate/terephthalate (PBAT) is a biodegradable plastic with excellent film-forming and processing properties. However, its poor mechanical performance and high cost limit its application in packaging. In this study, thermoplastic starch (PTPS) was first prepared by melt extrusion using a composite plasticizer, polyvinyl alcohol (PVA), and starch, and then blended with PBAT to produce PBAT/PTPS composite films via blow molding. The structure of the plasticized starch and the properties of the composite films were analyzed through FT-IR, XRD, and DSC. The results indicate that the composite plasticizer forms more stable hydrogen bonds with starch, resulting in a tensile strength of 2.59 MPa and an elongation at break of 76.67%. After adding 5% PVA, the tensile strength of PTPS is increased significantly by 32.43% compared to thermoplastic starch without PVA, while the elongation at break is slightly decreased by 7.96%. When the PTPS mass fraction is 6%, the tensile strength and elongation at break of the composite film are increased by 5.45% and 21.82%, respectively, compared to pure PBAT film, with a 2.21% increase in crystallinity. The water contact angle is decreased by 21.65%, and UV blocking at 400 nm drops by 40.32%. Research demonstrates that incorporating PTPS not only enhances the mechanical properties and crystallinity of PBAT while significantly reducing the water contact angle, but also lowers the production cost of PBAT.