Abstract: The transformation behavior of mineral matter in two coals (Xiaolongtan lignite and Fugu bitumite) with a high Si/Al ratio in coal ash was investigated under reducing atmosphere at high temperature by XRD and FT-IR. Furthermore, the theoretical changes of the main mineral composition at high temperatures were calculated using the thermodynamic software package FACT sage. It is found that the transformation behavior of mineral matter in the two high Si/Al ratio coals is different. The content of CaO has different effects on the transformation behavior of mineral. The major mineral matter formed at high temperature for the coal ash with high Ca content may be anorthite or gehlenite that maybe includes Mg, Na and K elements, while the major mineral formed for the coal ash with low Ca content maybe contains a large amount of aluminosilicate and a little anorthite. Iron oxide has little effect on the components of mineral matter in coal ash. The content of quartz decreases with increasing temperature. The forming of eutectic mixture leads to the decrease of the fusion temperature of coal ash, and the increase of the amorphous phase as well. The results of FACT sage calculation are consistent with the analysis of XRD and FT-IR.