Polylactic acid (PLA) films are limited in their further applications due to their high brittleness. To enhance the toughness of PLA-based films, UFBC modified with biocohesive agents (TA-ESO) was used, in this study, to improve compatibility and interlocking adhesion with PLA. Benefiting from the effects of TA-ESO in plasticization and interface improvement, TA-ESO-modified UFBC can make PLA more pliable while establishing a stronger mechanical interlocking structure, thus raising the mechanical properties of the system. Compared to pure PLA films, the tensile elongation of TA-ESO-modified UFBC/PLA composite films increased by 189.11%, and the tensile strength remained at a high level of 23.95 MPa, along with an increase in crystallinity (Xc) from 1.09 to 2.05%. Given their excellent strength and toughness, TA-ESO-modified UFBC/PLA composite films are expected to offer various potential applications in the packaging, medical, and agricultural fields.