With the gradual depletion of high-quality iron-ore resources, ironmaking blast furnaces have gradually shifted to using high-content Al₂O₃ iron ore. Under this condition, the addition of MgO in the slag is adopted to alleviate operation problems caused by the high content of Al₂O₃. However, the proper value of the MgO/Al₂O₃ ratio for blast-furnace slag systems (Al₂O₃ = 8–25%) is not systematically studied. In this paper, we discuss the proper MgO/Al₂O₃ ratio on the basis of blast-furnace slag systems under different Al₂O₃ contents. On the basis of thermodynamics and phase-diagram analysis, it could be concluded that: (1) the MgO/Al₂O₃ ratio is not limited when Al₂O₃ in slag is less than 14%, (2) the MgO/Al₂O₃ ratio is required to be in the range of 0.40–0.50 when Al₂O₃ in the slag is 15–17%, and (3) the MgO/Al₂O₃ ratio should be 0.45–0.55 when Al₂O₃ in the slag is larger than 18%. The proper MgO/Al₂O₃ ratio value has been established in various industries, and it has achieved significant economic and social benefits.