The invention discloses SiC-based composite ceramic and a preparation method thereof. The SiC-based composite ceramic is prepared from Si and SiC, wherein SiC is a continuous matrix phase, Si is a free phase, and the free phase is distributed in the continuous matrix phase on the surface of the SiC-based composite ceramic. The preparation method comprises the following steps that 1, a mixed solid phase, organic monomers, a cross-linking agent and water are mixed and then subjected to ball milling, pre-mixed slurry is obtained, the mixed solid phase is composed of carbon black and alpha-SiC ceramic powder, the pre-mixed slurry is subjected to vacuum degassing and then mixed with an azoic compound, and mixed slurry is obtained; 2, gel casting is carried out; 3, gaseous phase siliconing sintering is carried out. The SiC-based composite ceramic has the advantages of being high in density, high in heat conductivity, low in heat expansion coefficient, excellent in polishing performance and the like; the preparation method is short in period, low in sintering temperature, free of sintering shrinking, controllable in finished product performance and capable of adapting to net-size forming of SiC-based ceramic elements in complex shapes.