The fluidity of coal ash slag at high temperatures is an important factor that determines its applicability and operating conditions in entrained flow gasification. In this study, six synthetic slags with different CaO/MgO rations were prepared to study their effect on slag fluidity. The results showed that the flow temperature of slags decreased first and then increased with the decrease of CaO/MgO ratio. When the slag had a high or low CaO/MgO ratio, or when the contents of CaO and MgO in the slag were similar, the homogeneous slag showed a lower viscosity. The crystallization tendency of Ca was stronger than that of Mg. During cooling, slags with high or low CaO/MgO ratio showed obvious crystallization tendency, and formation of crystals would lead to a sharp increase in slag viscosity, so that the temperature of critical viscosity of slags decreased first and then increased with the decrease of CaO/MgO ratio.