The 3D graphite/ceramic composites prepared by selective laser sintering technology are porous and have poor mechanical properties and thermal conductivity. In this paper, the effects of different composition of mixed powder and different post-treatment process on thermal conductivity and compressive strength of graphite/ceramic composites were studied. The results show that the compressive strength and thermal conductivity of the composites are increased by adding high purity silicon powder to form silicon carbide reinforced phase at high temperature, and the compressive strength and thermal conductivity of the composites are decreased by adding expandable graphite powder due to the increase of closed porosity, but the latter decreases more obviously. The compressive strength and thermal conductivity of 3D graphite/ceramic composites were greatly improved by vacuum pressure impregnation of phenolic resin solution and silica sol solution. Finally, a 3D graphite/ceramic composite mold with compressive strength of 13.04 MPa and thermal conductivity of 1.94 W/(m·K) was prepared, which was successfully used in repeated casting of steel castings. Through the analysis of casting defects and use cost, the prepared graphite/ceramic composite parts are expected to replace the traditional sodium silicate sand mold.