Abstract: In order to improve the electrical properties of epoxy resin for terminal insulation of superconducting DC power transmission system, the Al₂O₃epoxy composites with mass fraction of 1% and particle size of 50nm, 1μm and 10μm were respectively prepared. The electrical tree initiation characteristics of composite dielectrics at room temperature and liquid nitrogen temperature under DC voltage, and the electrical tree growth process and partial discharge characteristics under DC 35kV voltage were studied. The trap characteristics of Al₂O₃ epoxy resin composites were analyzed by surface potential decay tests. The results show that at room temperature and liquid nitrogen temperature, micron and nanometer Al₂O₃ can inhibit the initiation and growth of electrical tree in the epoxy resin. Moreover, the smaller the particle size of Al₂O₃, the better its inhibition effect on the electrical treeing. Liquid nitrogen environment has an obvious effect on the morphology of the electrical tree in epoxy resin and its composites. Liquid nitrogen at low temperature can significantly inhibit the growth rate of electrical tree in the materials, but there is a phenomenon that low temperature leads to cracking and breakdown of the insulating materials. Finally, the trap characteristics of the composites were calculated according to the surface potential decay curve. The analysis of trap characteristics shows that the doping of micro/nano Al₂O₃ would change the density and energy level of the traps in the material, thus changing the dielectric properties of the materials.