Several boron nitride (BN)/aluminum oxide (Al₂O₃) composite powders were synthesized by liquid-solid phase co-mixture method. BN was modified by the freeze-thaw method and surface modification. The precursor and as-sintered BN/Al₂O₃ composite powders were obtained by changing the type of surface modification materials and the molar ratios. Additionally, the thermal conductivity of the epoxy resin (EP)-based BN/Al₂O₃ composite materials prepared by mechanical mixture was measured and analyzed. Results show that the dispersion and interfacial compatibility of BN after freeze-thaw process are obviously better than those without freeze-thaw process. The modification effect of dopamine on BN is better than that of polyethylene glycol. When the dopamine is used as the surface modification material and the molar ratio of BN to Al(NO₃)₃ is 1:1, BN powder at micro-level with uniform nano-Al₂O₃ deposition can be obtained, namely the BN/Al₂O₃ micro/nano-composite powder. The thermal conductivity coefficient of this EP-based BN/Al₂O₃ composite reaches 0.62 W·m^-1·K^-1, which is 3 times and 1.5 times higher than that of the pure EP composite and EP-based composite material prepared by pure micro-sized BN powder, respectively. The deposited nano-Al₂O₃ on BN surface can form multiple thermal conduction paths, thereby improving the thermal conductivity of the EP-based BN/Al₂O₃ composites.