Rapid development of high-power electronic equipment for 5G and other advanced communication devices leads to a highly compact component size with increased heat flux density in integrated circuits, which requires electronic packaging materials to meet excellent heat dissipation performance. In this work, a novel liquid-solid separation technology was used to prepare a 40% (volume fraction) diamond/Al composite for electronic packaging substrates. By SEM, EPMA, and XRD techniques to investigate the fracture morphology and interface structure of the composite, the influence of diamond particle sizes (90, 106, 124, and 210 μm) on the thermophysical properties of diamond/Al composite was studied. The results showed that with the increase of a diamond particle size, the density of a composite material increased first and then sharply decreased and attained the optimal value when the diamond particle size was 106 μm. No harmful intermetallic Al₄C₃ was generated at the interface of the composite material. Coefficient of thermal expansion ({\alpha }_{\mathrm{c}}) of the composite material increases slightly with increase in diamond particle sizes and remains relatively stable. The Kerner model can accurately simulate the {\alpha }_{\mathrm{c}} of the diamond/Al composite. Thermal conductivity (λ) is also affected by diamond particle sizes and interface behaviors, and the trend of change in λ with the size of the diamond particles follows a similar trend to that between the density of composites and the particle size. The diamond/Al composite with diamond particle size of 106 μm has the best overall performance, relative density and {\alpha }_{\mathrm{c}} attained 97.12% and 12.4 × 10^-6 K^-1, respectively. λ is 153.1 W/(m·K), meeting 69.31% of the Maxwell-Eucken model prediction value. The airtightness value meets the military standard for this type of an electronic packaging material.