N-Acetyl cysteine (NAC) is an essential molecule that boosts acute lung injury (ALI) defense via its direct antioxidant capability. Nevertheless, the therapeutic use of NAC is limited due to its poor bioavailability and short half-life. In this study, NAC was grafted to the polyurethane consisting of poly(propylene fumarate), poly(thioketal), and 1,6-hexamethylene diisocyanate (PFTU) to reduce excessive oxidative stress and inflammatory factors in ALI. The NAC-grafted polymer nanoparticles (NPT@NPs) were prepared as a drug delivery system, which could effectively scavenge free radicals and reduce inflammation in vitro. The administration of NPT@NPs exhibited notable efficacy in ameliorating pulmonary edema, attenuating the presence of inflammatory cells, suppressing myeloperoxidase expression, diminishing the levels of pro-inflammatory cytokines, and reversing cell apoptosis in an ALI model induced by lipopolysaccharide (LPS). The NPT@NPs demonstrated significantly better efficacy compared to the free NAC in mitigating the deleterious effects of LPS on pulmonary tissue, thereby providing more effective protection against pulmonary inflammation.