Using agricultural wastes (bagasse) and polyol derived from soy oil as raw materials to develop biodegradable rigid polyurethane foams (RPUFs) is beneficial to reduce the dependence on petroleum resources and promote the sustainable socioeconomic and environmental development. This study focuses on bagasse as a functional filler to improve and control the mechanical properties and biodegradability of the RPUFs. With the increasing levels of bagasse content, the density and mechanical properties of RPUFs increase. The biodegradation of RPUFs has a greater enhancement compared to the foam without bagasse, and all bio-foams have excellent thermal insulation properties. All changes in foam performance are due to the fact that the active hydroxyl groups on bagasse react with excess isocyanate to form urethane, which affects the structure of RPUFs. In this study, scientific design and molecular regulation theory were applied to improve the utilization value of bagasse and develop high-performance biodegradable RPUFs for thermal insulation materials.