Abstract:R.S.McCaffery’s study of influence of MgO on slag viscosity is pioneering, and has been an important reference for the production and research of slag for many years. But in condition of MgO>15%, the conclusion that the change of slag composition has no effect on slag viscosity is defective. According to the slag compositions of 125 blast furnaces from 63 ironworks in China, Japan, South Korea, the United States, Australia, Europe and the former Soviet Union, the corresponding relationship between the basicity of slag and the ratio of Al2O3/MgO has been analyzed. As a result, the slag basicity and the corresponding ratio of Al2O3/MgO are obtained which can provide references for production and research.