In order to investigate the structure and properties of Al₂O₃/polyolefin composites, spherical Al₂O₃ was pretreated with a silane coupling agent, and high density polyethylene (HDPE)/polypropylene (PP) composites were prepared by primary mixing and melt blending with pretreated Al₂O₃ as filler and high density polyethylene (HDPE)/polypropylene (PP) mixture (mass ratio 5∶95) as matrix. The investigation of the influence of the mass fraction of Al₂O₃ on the structure and properties of the composites was studied. The results showed that the addition of Al₂O₃ significantly enhanced the thermal conductivity of the composites. Specifically, the thermal conductivity increased proportionally with the mass fraction of Al₂O₃. When the Al₂O₃ mass fraction reached 25%, the thermal conductivity of the composite measured 0.331 W/(m∙K), which was 231% higher than that of the pure polyolefin matrix material. Moreover, the presence of Al₂O₃ particles with high filler content led to a noticeable reduction in gaps within the composite, thereby facilitating the construction of thermal conductivity pathways. Additionally, the incorporation of a lower content of rigid Al₂O₃ particles improved crystallization ability of the polymer. Notably, the addition of the spherical Al₂O₃ also significantly enhanced the melt flow rate of the composites, and improved its processing performance.