The chemical composition, phase constituents and microstructure of gasification slag from Texaco gasifier, the carbothermal reduction nitridation of gasification slag were investigated by X-ray fluorescence spectrometry, X-ray diffractometry and scanning electron microscopy. The effect of nitridation temperature on the phase composition and morphology of nitridation reaction products was studied. Ca-α-sialon—SiC multiphase ceramics were fabricated and characterized. The results showed that: The coal gasification slag was an ideal raw material to synthesize Sialon powder. When introducing 3wt%Y2O3+2wt% MgO into sialon powder carbothermally synthesized at 1450°C, the Vickers hardness and fracture toughness of Sialon-SiC multiphase ceramics hot-pressed at 1650 °C were 18.6 GPa and 5.2 MPa·m1/2, respectively.