The urgent demand for high-temperature dielectrics toward capacitive energy storage arises from numerous emerging harsh-environment high-temperature applications such as electric vehicles, aerospace power conditioning, downhole oil explorations. For these applications, it is imperative to enhance the discharge energy density of polymer-based dielectrics at elevated temperature. Herein, a trace composite approach is proposed and demonstrated. Two types of polyetherimides (PEIs)-based dipolar glasses are designed, synthesized and composited with trace amount of ultrafine Al₂O₃ nanoparticles. PEIs-based dipolar glasses with high T_g (∼250 ℃) and much enhanced electrical polarization are constructed by introducing dipolar groups (-CF₃) and sulfonyl groups (-SO₂-) with high dipolar moment (∼4.30 D) in the polymer chains. The incorporation of ultrafine Al₂O₃ nanoparticles increases the electrical polarization and keeps low loss. The remarkable permittivity enhancement via the ultrafine filling could be ascribed to more active mobility of dipolar groups characterized by activation energy of movement and the correlation of arrangement of dipolar groups in the chains with this effect is also revealed. These results will contribute to deeply understanding and rationally designing high-performance polymer-based nanocomposites orienting to high-temperature electrical energy storage.