α-Al₂O₃ was prepared using the molten salt method, and its surface was modified with the silane coupling agent γ-aminopropyltriethoxysilane (KH550), resulting in KH-α-Al₂O₃. Using low-density polyethylene (LDPE) as the matrix and different α-Al₂O₃ as additives, grounding wire insulation materials of α-Al₂O₃/XLPE and KH-α-Al₂O₃/XLPE were prepared. The crystal structure, microstructure, and functional groups of α-Al₂O₃ and KH-α-Al₂O₃ were analyzed, and the effects of different α-Al₂O₃ contents on the electrical and thermal properties of α-Al₂O₃/XLPE grounding wire insulation materials were investigated. The results show that the α-Al₂O₃ prepared by the molten salt method has a hexagonal structure, high crystallinity, and is an irregular disc with a diameter of 5~10 μm. The modification with KH550 does not alter the crystallinity of α-Al₂O₃. In the KH-α-Al₂O₃/XLPE grounding wire insulation material, KH-α-Al₂O₃ is aligned along the thickness direction of the material and is more uniformly dispersed, which is beneficial for suppressing charge transport within the matrix. KH-α-Al₂O₃ enhances the DC breakdown strength and DC conductivity of XLPE. When the mass fraction of KH-α-Al₂O₃ is 1.5%, the DC breakdown strength of KH-α-Al₂O₃/XLPE reaches 320 kV/mm, and the conductivity is 1.043×10⁻¹³ S/m. The introduction of KH-α-Al₂O₃ creates a certain number of traps at the interface between the XLPE matrix and KH-α-Al₂O₃, and effectively suppresses the injection of charges. KH-α-Al₂O₃ significantly reduces the thermal degradation rate of XLPE and improves the thermal stability of the KH-α-Al₂O₃/XLPE grounding wire insulation material. When the mass fraction of KH-α-Al₂O₃ reaches 1.5%, the decomposition temperature increases to 475.44 ℃, and the thermal conductivity under 90 ℃ conditions increases from 0.390 W/(m·K) to 0.545 W/(m·K).