Abstract: To address the poor degradability and severe residual pollution associated with conventional polyethylene (PE) mulch films, a series of fully biodegradable mulch films (PLA/OL/LG) were developed via the synergistic modification of a polylactic acid (PLA) matrix using biomass-derived alkali lignin (LG) and glyceryl tripalmitate (OL). The resulting films were systematically characterized in terms of their microstructure, mechanical properties, thermal behavior, and degradation performance to identify the optimal formulation. The results demonstrate that the synergistic incorporation of appropriate amounts of OL and LG significantly enhances the toughness of PLA, achieving a favorable balance between stiffness and toughness while maintaining good thermal stability and barrier properties. Compost degradation tests reveal that the modified mulch films exhibit a high biodegradation rate, indicating excellent environmental compatibility. Field application trials further show that, compared with conventional PE mulch films, the modified PLA-based biodegradable mulch films display good adaptability to plant growth and exhibit potential improvements in the coordination of chemical constituents and intrinsic quality of tobacco leaves. Sensory evaluation results also confirm that the application of the modified PLA biodegradable mulch films contributes to the maintenance and enhancement of tobacco leaf sensory quality. Economic assessment indicates that the cost of the modified PLA biodegradable mulch films is comparable to that of traditional PE mulch films, demonstrating promising application feasibility. Therefore, the modified PLA-based fully biodegradable mulch films provide a viable technological pathway for the development and application of high-performance, environmentally friendly green mulch materials.