Abstract: Coal ash and synthetic ash samples were used to detect effect of different ash components on ash fusion temperatures (AFTs). Thermodynamic database FactSage 7.0 was applied to simulate the melting process of ashes with different compositions, in order to provide theoretical basis for effect of ash components on ash fusibility. The reducing effect of Na₂O on AFTs is due to the replacement of anorthite by albite and nepheline. The increasing content of MgO can initially lower and then raise the AFTs. When MgO exceeds certain content, forsterite is generated and raise the AFTs. The raising effect of sulfur on AFTs is due to the replacement of diopside by forsterite and calcium sulfate. The increasing content of CaO can also initially lower and then raise the AFTs. When CaO exceeds certain level, Si migrates from minerals with low melting points to those with higher melting points, thus raising the AFTs. Na₂O is prior to CaO when being bound with silica-oxygen units to form minerals. The priority order of the oxides when being bound with CaO and silica-oxygen units is: Al₂O₃ > MgO > Fe₂O₃.