Polylactic acid ( PLA) / modified polymerized rosin ( PR-g-OLA₁₀-N⁺) antibacterial fibers with different mass ratios were successfully prepared by melt spinning technology. The quaternary ammonium salt cations (N⁺) and oligolactic acid chain segments were successfully grafted onto the framework of polymer rosin (PR) by chemical grafting method. Differential scanning calorimetry ( DSC) and X-ray diffraction ( XRD) analyses indicate that the modifier with low mass fraction (≤2% ) increases the crystallinity of PLA to 32.55% through heterogeneous nucleation, while maintaining good thermal stability (melting temperature of 158.1 ℃) at high mass fraction (5% ). Mechanical property tests show that the addition of 2% modified rosin reduces the fiber breaking strength by approximately 15% compared to pure PLA, but significantly increases the elongation at break by 23% , and the material toughness is effectively improved. Observation by scanning electron microscopy (SEM) shows that when the mass fraction of modified rosin is 5% , quaternary ammonium salt cations are uniformly distributed on the fiber surface, and the antibacterial rates against Escherichia coli and Staphylococcus aureus both exceed 99% . This study aims to develop polylactic acid-based composite fibers with both excellent mechanical properties and efficient antibacterial activity, and to investigate the influence of the addition amount of modified polymerized rosin on the crystallization behavior, thermal stability and antibacterial effect of materials.