Polymer film capacitors play a pivotal role as core energy storage components in new energy vehicles, power electronics, oil and gas exploration, and aviation electrification equipment. However, as equipment evolves towards higher power density and operation in extreme environments, the energy storage performance of existing polymer dielectrics significantly declines at elevated temperatures, becoming a critical limiting factor for the further development of film capacitors. Therefore, the development of novel dielectric material systems that combine high heat resistance with excellent energy storage performance is particularly crucial. Polyetherimide (PEI) is regarded as a highly promising polymer dielectric for high-temperature energy storage applications at 200°C and above, owing to its high glass transition temperature, exceptional thermal stability, and dielectric properties. Nevertheless, as temperature rises, issues such as intensified thermal motion of polymer chains, conformational rearrangement of aromatic segments, and π–π stacking between benzene rings emerge.