It is worth to study the synthesis of β-Sialon from coal gangue, because coal gangue is a waste of coal production andis a high quality kaolin contained carbon which is a perfect raw material of contained reducer itself for synthesis of β-Sialon. The study showed that a high conversion rate of 95% from coal gangue to β-Sialon could be obtained by usingprocess of carbothermal reduction nitridation when strictly controlled the thermodynamic conditions of synthesis. Forcontrolling the synthesis conditions easy, the details of the effects of Pco, Po2 and T on the conversion rate of β-Sialon were discussed detailedly and the phase diagrams of oxygen pressure vs composition for Si3N4-AlN-Al2O3-SiO2system at 1350, 1500, and 1600℃ were constructed in the paper.