By the gelcasting process, the porous graphite preform prepared by selective laser sintering is mixed with mullite ceramic slurry, and after freeze drying and sintering, the graphite/mullite composite is obtained. The performance of each sample is tested and analyzed, and the suitable sintering process is determined. The results show that the open porosity, density, thermal conductivity, and bending strength of the porous graphite preform are 60%, 0.518 g/cm 3, 1.01 W·m -1·K -1 and 1.7 MPa, respectively. For the sintered mullite ceramics, the relative density is 80.4%, the compressive strength is 28.3 MPa, the bending strength is 27.5 MPa, and the thermal conductivity is 2.35 W·m -1·K -1. The post-processing of porous graphite preforms can avoid the occurrence of cracks in the combination zone and the poor combination of two phases, and the integrity and interfacial bonding of composites are greatly enhanced.