Abstract: The filling of micron-sized Al₂O₃ powder into epoxy resin matrix can improve the thermal conductivity of composites, but the interfacial incompatibility between the powder and matrix can facilitate the degradation of insulating properties of epoxy composites. In this paper, a rotatable drum-type dielectric blocking discharge modification device was used to improve the insulating property of epoxy composites by using hexamethyldisiloxane as the medium, and plasma was used to introduce silicone-containing film on the surface of the powder. The breakdown field strength of the composites prepared after plasma treatment for 25 min was 41.5 kV/mm, which was 28.9% higher than that (32.2 kV/mm) of the pure EP material, and 39.2% higher than that of the composites with untreated powder. The insulating properties of the treated epoxy composites were significantly improved while maintaining high thermal conductivity. The treatment method provides a reference for solving the compatibility problem at the powder-matrix interface.