In recent years, more and more high Al2O3 iron ores were used in East Asia, especially in China, which increased the Al2O3 content in blast face (BF) slag and resulted in poor metallurgical properties of slag. Adding MgO-bearing flux is one of the methods to improve the metallurgical properties of slag with high Al2O3. However, there is lack of theoretical basis for the proper MgO/Al2O3 ratio. Therefore, the properties of slag in SiO2–CaO–MgO–Al2O3 system were investigated based on viscosity experiments and phase diagram analyses at 1500℃, and the proper MgO/Al2O3 ratio was explored according to the Al2O3 content in slag. The experimental results show that: (1) in the case of Al2O3 content less than 14mass％, there is no limitation of the MgO/Al2O3 ratio, and the amount of MgO in slag can be determined according to the hearth tempera-ture and desulfurization; (2) in the case of Al2O3 content between 15 and 17mass％, the proper MgO/Al2O3 ratio should be 0.40–0.50; (3) in the case of Al2O3 content between 18 and 20mass％, the proper MgO/Al2O3 ratio should be 0.45–0.55. Consequently, the three-segment control theory of MgO/Al2O3 for BF slag was built, and the actual BF performances proved the validity and applicability of this theory. The research contents and results can give theoretical guidelines for stable BF operation in a wide range of Al2O3 content of 12–20mass％.