To alleviate the environmental pressure caused by waste cotton straw and improve the utilization of cotton straw by-products, ethylene glycol solvent was used to separate lignin from cotton straw, the effects of various factors on its separation effect were studied, and the properties of the extract and its effect on the antioxidant capacity of polypropylene (PP) were analyzed. The results show that the highest yield of 84.58% could be achieved by treating cotton straw with 0.8 mol/L ethylene glycol solution for 1 h at 160 ℃. The lignin modification has little effect on the crystallization properties, and melting and crystallization temperatures of PP, but has an effect on its oxidation induction time, rheological properties, and mechanical properties. When 0.5% of lignin was added to PP, the oxidation induction time is increased by 38.3% compared with that of pure PP, the shear torque is 25.58% higher in micro mixing rheological test, the elongation at break is 9.34% lower, and the impact strength has no change, indicating that the lignin extracted by ethylene glycol method can effectively exert an antioxidant effect in PP.