Nine different fusion temperature coals were selected, and the relationship between ash fusion temperature and ash mineral composition and content were studied by using XRD. The mineral transformation of typical coal ashes in reducing atmosphere was also investigayed. The results show that the mineral phases of coal ashes which are ashed at 815℃ mainly include quartz, anhydrite, hematite, portlandite, lime and potash feldspar. Higher anhydrite and hematite content and lower portlandite content would result in lower ash fusion temperature. As the heating temperature increases, the content of quartz, anhydrite and portlandite in coal ash decreases in reducing conditions and some new mineral phases are formed. Mullite is formed, which leads to higher ash fusion temperature. However, the formation of anorthite and hedenbergite at 1100℃ in the heating process is the main reason for the ash melting temperature lowing.