In this study, polylactic acid (PLA)/modified polymerized rosin (PR-g-OLA10-N+) antibacterial fibers with different ratios were successfully prepared by melt spinning technology. The quaternary ammonium salt cations (N+) and oligolactic acid chain segments were successfully grafted onto the polymer rosin (PR) framework by chemical grafting method.Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses indicated that the modifier with a low addition amount (≤2 wt%) increased the crystallinity of PLA to 32.55% through heterogeneous nucleation, while maintaining good thermal stability (melting temperature 158.04°C) at a high addition amount (5wt%). Mechanical property tests show that the addition of 2wt% 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) showed that when the addition amount of modified rosin was 5wt%, quaternary ammonium salt cations were uniformly distributed on the surface of the fibers, and the antibacterial rates against Escherichia coli and Staphylococcus aureus both exceeded 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 the materials.