Abstract: Aluminum oxide is a common component in mold powder and is a kind of amphoteric oxide. It shows the characteristics of acid oxide under high-alkalinity conditions and of alkaline oxide under low-alkalinity conditions. In general, adding Al₂O₃ to the traditional CaO–SiO₂-based mold flux will increase the viscosity and melting point of the mold flux, which will consequently reduce the mold flux’s ability to adsorb inclusions. In addition, as the content of Al₂O₃ in the slag increases, the solidification temperature of the slag can be reduced, thereby improving the lubricating ability of the mold flux. At present, the research on the crystallization performance of Al₂O₃ on mold fluxes mainly focuses on low-reactivity or non-reactive mold fluxes for high-aluminum steel and high-titanium steel. Relevant studies have shown that Al₂O₃ in low-reactivity or non-reactive mold fluxes can increase the crystallization incubation time of the mold flux, reduce the critical cooling rate of the flux, and inhibit the crystallization process of the flux. In mold powder with low to medium alkalinity content (R = 1.2–1.5) or new CaO–Al₂O₃-based low-reactivity mold powder, the addition of Al₂O₃ will increase the viscosity of the slag and melting point and decrease (or increase) the solidification temperature and crystallization performance. In recent years, ultrahigh-alkalinity mold powder (R = 1.65–1.85) has been successfully applied in peritectic steel continuous casting mold powder, effectively coordinating the contradiction between the mold powder heat transfer and lubrication function. However, there is no relevant report on the influence of Al₂O₃ on the performance of mold flux under ultrahigh-alkalinity conditions. In this study, an ultra-high-alkalinity mold flux (comprehensive alkalinity R = 1.75) is taken as the research object, and the influence of Al₂O₃ on the flow, melting, and solidification characteristics of the mold flux is analyzed. The research results show that as Al₂O₃ increases, the viscosity and melting temperature increase, and the transition temperature decreases. Particularly, with an average increase of 1% Al₂O₃, the melting temperature of the mold flux will increase by approximately 5 ℃, and the turning temperature will decrease by approximately 12 ℃. In addition, as the Al₂O₃ content in the slag increases by 1%, the starting crystallization temperature drops by approximately 11 °C on average. The average crystallization rate decreases with the increase in Al₂O₃ in the slag, and Al₂O₃ has a significant effect on the crystallization rate. Moreover, with the increase in the content of Al₂O₃ in the slag, the proportion of crystals in the crystalline phase of the mold slag gradually decreases, but the type of crystals remains unchanged.