Abstract: A series of lignin-modified castor oil-based waterborne polyurethanes were synthesized using tetrahydrofuran, isophorone diisocyanate, and castor oil as the primary raw materials. Lignin was employed as a modifying agent through hydrothermal treatment of lignocellulose, and 2,2-dihydroxymethylpropionic acid was utilized as a hydrophilic chain extender. The pre-polymerization method was adopted for the preparation of these lignin-modified castor oil-based waterborne polyurethanes (LCWPU). The structural characteristics, thermal properties, crystallization, hydrophobicity, and glossiness of the LCWPU films were investigated. The effects of lignin content and the molar ratio of n_NCO/n_OH on the thermal stability, mechanical properties and water resistance of LCWPU films were investigated. The results indicate that the LCWPU films had an amorphous structure. With the increase of lignin content and n_NCO/n_OH molar ratio, the water absorption of the LCWPU films decreased and the tensile strength increased significantly. Notably, when the lignin content was 3% and the n_NCO/n_OH molar ratio was 4.5, the LCWPU film demonstrated the highest water resistance and tensile strength, with a contact angle of 95.7°, a water absorption of 14.3%, a tensile strength of 62.9 MPa, and an elongation at break of 297.5%. The designed LCWPU has good application prospects in the fields such as leather finishing agents and matte wood paints.