Abstract: In order to achieve reliable low-temperature brazing of high-thermal-conductivity graphite and aluminum alloy (5A06), Sn-Pb solder was used for brazing, and its spreading behavior on the base material was studied. The results show that metallization of the high-thermal-conductivity graphite surface effectively improves the wettability between metal and graphite, mitigates interfacial issues caused by mismatched thermal expansion coefficients, and reduces interfacial thermal resistance due to phonon scattering. Under optimized process conditions—Ag-Cu-Ti solder layer thickness of 0.2 mm, brazing temperature of 860 ℃, and holding time of 10 min for the metallization process, along with Sn-Pb solder brazing at 210 ℃ with a holding time of 15 min—the overall thermal conductivity of the composite structure was significantly enhanced. Furthermore, a heat spreader was designed and tested to evaluate its thermal performance. Experimental results show that a strip-shaped heat spreader with dimensions of 210 mm× 25 mm× 3.5 mm achieves a thermal conductivity of up to 558 W/(m·K), while a larger heat spreader with dimensions of 233.4 mm× 200 mm× 24 mm, suitable for large-scale integrated electronic devices, reaches a maximum thermal conductivity of 460 W/(m·K). This study not only demonstrates the potential of brazing technology in improving thermal management efficiency but also provides a novel material solution for the thermal management of high-performance electronic devices.