Abstract: In order to study effect of CaO content on the ash fusibility of high sodium coal, high sodium synthetic coal ash samples with different CaO content were prepared and ash fusion temperatures (AFTs) were measured. Thermodynamic database FactSage 7.0 was applied to simulate the ash melting process, and the mineral conversion was analyzed. The minerals' composition and surface morphology of synthetic coal ash samples were investigated by X-ray diffraction (XRD) and scanning electronic microscope (SEM). The results show that the AFTs drop first and rise later with the increase of CaO content. CaO addition influences the existence form and relative mass of both calcium-containing and sodium-containing minerals. At 1 000℃, the calcium-containing minerals transform from anorthite to andradite, wollastonite, gehlenite, rankinite and larnite in turn. The sodium-containing minerals transform from albite to nepheline. The results of binary phase diagram and analogous ternary phase diagram indicate that variation of liquidus temperature is consistent with the AFTs with increasing CaO content. When the CaO content is higher than 40%, the AFTs could be improved efficiently for the coal sample.