With increasing demands for renewable materials applications, biomaterials originated from natural resources have obtained great attention. Among them, plant proteins have been extensively studied for composites, packaging as well as biomedical applications. Despite the diverse and complex polar structures, plant proteins have rarely been studied for energy related applications. In this study, soy protein isolate (SPI), a high protein content product extracted from soy flour, was used to modify energy storage performances of poly (ethylene oxide) (PEO). PEO is a type of soluble and biodegradable thermoplastic polymer with good biocompabitlity and low toxicity, and it is usually considered as an ideal candidate for environment friendly applications. The pure PEO membranes are highly polarizable, but they also have extremely high energy loss during charging/discharging cycles. The high loss is not only responsible for the unsatisfactory energy storage performance, but also responsible for high dielectric heating. The addition of SPI to PEO leads to greatly reduced polarization and stored energy density determined by monopolar hysteresis analysis. However, it also largely reduces the current leakage and energy loss of the resulting membranes, leading to significantly enhanced discharged energy density and energy efficiency. The strong interactions between PEO and SPI should be the primary reason for the improve energy storage properties. Meanwhile, such interactions also result in a more brittle fracture behavior and reduced crystallinity of the PEO/SPI membranes. The enhanced discharged energy density and low energy loss suggest that PEO/SPI membranes are promising dielectric materials in high efficiency capacitor applications.