Abstract: Lignin is the most abundant renewable aromatic resource in nature. Utilizing lignin to develop multifunctional biobased plastics is expected as one of the most effective ways to alleviate plastic pollution and reduce the dependence on petroleum resources. This study innovatively proposed a cellulose acetate (CA) toughening system to regulate the structure and properties of lignin/polybutylene adipate-co-terephthalate (PBAT) composite films and compared to the traditional triethyl citrate/methylene diphenyl diisocyanate (TEC-MDI) system, aiming to solve the problem of decrement in the mechanical properties of PBAT with high lignin loading. A solution casting method combined with hot-pressing was developed to prepare lignin/PBAT composite films. Accompanying with lignin, cellulose acetate (CA) or triethyl citrate (TEC)/diphenylmethane diisocyanate (MDI) was introduced. The toughening mechanisms were investigated through tensile tests, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), ultraviolet blocking, and gas barrier performance tests. The results demonstrated the superiority of CA over the TEC-MDI system. The film with mass fraction 20% lignin showed poor mechanical properties (Elongation at break: 12%±2%; Tensile strength: (2.6±0.2) MPa). After toughening with mass fraction 15% CA, the interfacial compatibility maybe enhanced via hydroxyl-ester hydrogen bonds, leading to uniform dispersion of lignin and a dense fracture surface. Consequently, the elongation at break and tensile strength recovered to 430%±10% and (11.2±0.6) MPa, respectively. The incorporation of 20% lignin also provided excellent UV absorption, with UVA and UVB blocking rates reaching to ~100%, while the gas permeability remained comparable to that of pure PBAT. This CA toughening strategy effectively addresses the mechanical deterioration caused by poor interfacial compatibility in PBAT/lignin composite films, providing a technical foundation for preparing high-performance and multifunctional biodegradable films.