This paper investigated the effect of the pressures, reaction atmospheres and coal ash species on the ash fusibility with high-pressure thermogravimetric analysis (PTGA) apparatus and X-ray diffraction (XRD) analysis. Each specimen analyzed by XRD was observed for the mineral conversion and formation of new minerals with the pressures under different atmospheres. These results indicate that the pressure restrains the transformation and decomposition of minerals. Many low-temperature minerals are still present under the elevated pressure. The different reaction atmospheres have different effects on the formation of coal ash minerals. Under the N₂ atmosphere, the present microcline may decrease the melting temperature of coal ash. And later, it transforms into sanidine at high pressure; thus, the melting temperature of coal ash may increase. Under the CO₂ atmosphere, the minerals such as microcline, lomonitite, geothite and illite are still present with the increase in pressure; this may reduce the melting temperature. While under the H₂O atmosphere, there are magnetite and anorthoclase, which may produce the low-temperature eutectics decreasing the melting temperature. The coal ash abundance in basic oxides or higher SiO₂, Fe₂O₃, K₂O and Na₂O has lower melting temperature. While the ash sample with more SiO₂ and Al₂O₃ and less Fe₂O₃ and basic oxides may lead to higher melting temperature.