Compared with Cu/Al₂O₃ composites, high-strength Cu/Al₂O₃ composites usually exhibit obviously deteriorated electrical conductivity. A chemical and mechanical alloying-based strategy was adopted to fabricate ultrafine composite powders with low-content reinforcement and constructed a combined structure of Cu ultrafine powders covered with in-situ Al₂O₃ nanoparticles. After consolidation at a relatively lower sintering temperature of 550 ℃, high-volume-fraction ultrafine grains were introduced into the Cu/Al₂O₃ composite, and many in-situ Al₂O₃ nanoparticles with an average size of 11.7±7.5 nm were dispersed homogeneously in the Cu grain. Results show that the composite demonstrates an excellent balance of high tensile strength (654±1 MPa) and high electrical conductivity (84.5±0.1% IACS), which is ascribed to the synergistic strengthening effect of ultrafine grains, dislocations, and in-situ Al₂O₃ nanoparticles. This approach, which utilizes ultrafine composite powder with low-content reinforcement as a precursor and employs low-temperature and high-pressure sintering subsequently, may hold promising potential for large-scale industrial production of high-performance oxide dispersion strengthened alloys.