Ash fusion temperatures (AFTs) are important parameters influencing the stable operation of industrial entrained-flow gasifier. It is well known that the separate addition of CaO and MgO can ameliorate the ash fusibility. However, the synergistic effect of CaO and MgO on the ash melting characteristics is still unclear. For this sake, the ash fusion temperature analyzer (AFTA), X-ray diffraction (XRD), Raman spectrum, and FactSage thermodynamics software were applied to investigate the ash fusion behavior with various CaO/MgO mass ratios in this work. It showed that as the CaO/MgO mass ratio increased, the AFTs decreased first and then increased slightly with reaching the minimum at the CaO/MgO mass ratio of 8:2. When the CaO/MgO mass ratio was low, the formation of spinel with stable lattice was the main reason for the high AFTs. As the CaO/MgO mass ratio increased, the formation of low-melting-point feldspar minerals such as anorthite and melilite resulted in the AFT reduction. As well, when the mass ratio of CaO/MgO reached 8:2, the relatively high amorphous content led to the lowest AFTs. Additionally, the variation of the liquid slag content with the increase in temperature was analyzed, and the melting process was divided into the primary fusion stage and the free-liquid stage.