The carbon conversion of fluidized bed gasification has been restricted by the entrainment of semichars with high carbon content. Recently, the disposal of the semichar has become an intractable problem. In this study, the ash fusion properties and mineral transformation behavior at high temperature of the semichar from an industrial circulating fluidized bed (CFB) gasifier were investigated under oxidizing condition. In comparison to the raw coal, the total base content increases from 27.92 to 29.45 after partial gasification, which results in a decrease in the ash fusion temperatures (AFTs) of the semichar. Kaolinite decomposed into quartz, alumina, and water via dihydroxylation reaction and anhydrite was transformed into oldhamite in the CFB gasifier. As the particle size increases, the AFTs of the semichar decrease first and then remain almost unchanged, which coincides with the variations in acid-to-base ratio; however, the disparity is relatively small, which favors its melting utilization. Quartz, anorthite, pyroxene, and hematite are the main crystalline phases of the semichar ash at high temperature, and the major molten phase turns out to be anorthite. The liquidus temperature (1325 degrees C) predicted by FactSage is slightly higher than the flow temperature (1310 degrees C) of the semichar ash due to kinetic limits and mass transfer. In situ observation of the semichar ash melting process indicates that low-melting-point minerals first fuse and flow between the adjacent particles; as the temperature increases, high-melting-point minerals then start to melt, the molten-phase content increases, and the flow behavior accelerates.