Bio-based cross-linked polyurethane films were developed from stevioside as a multifunctional natural polyol and L-lysine diisocyanate via a feed ratio-controlled strategy. The influence of monomer composition on network architecture, physicochemical properties, and functional performance was systematically investigated. Structural analyses confirmed successful urethane formation and revealed that increasing diisocyanate content enhanced crosslink density, resulting in reduced swelling, reduced surface wettability, and improved thermal stability. Degradation studies demonstrated tunable behavior, where lower crosslinked networks exhibited higher enzymatic degradability, while densely crosslinked structures showed greater stability. To impart biofunctionality, resveratrol was incorporated into the polymer network, leading to a significant enhancement in antioxidant activity (up to 81.3% radical scavenging) and antibacterial performance against both Gram-positive and Gram-negative bacteria. Notably, films with intermediate crosslink density exhibited the most effective antibacterial activity, indicating an optimal balance between bioactive compound retention and diffusion-controlled release. In vitro cell culture studies demonstrated good biocompatibility, effectively supporting fibroblast adhesion, proliferation, and cytoskeletal organization, with no detectable cytotoxicity upon resveratrol incorporation. Collectively, these findings indicate that stevioside-based polyurethane films represent a versatile and sustainable platform with tunable structural and functional properties, which may be promising for further investigation in biomedical and active material applications.