To understand the thermodynamic characteristics of the reaction between Al and SiO 2 during the continuous casting process of the high-aluminum steel, the activity coefficients of SiO 2 and MnO in molten slags of (18-43%) CaO-(33-64%) SiO 2 -(9-21%) Al 2 O 3 -(2-3%) MgO-( < 2.4%) MnO were measured at 1 400°C, 1 450°C and 1 500°C by the experiments of the Si and Mn equilibrium between liquid copper and molten slag in the graphite crucible under the mixed gas atmosphere of CO and Ar. The effects of SiO 2 , Al 2 O 3 , the basicity, the radio of CaO/Al 2 O 3 and the temperature on the activity coefficients of SiO 2 and MnO in molten slag were discussed. The quadratic regression relationships among the activity coefficient of SiO 2 or MnO, the concentration of component, the basicity and the temperature were investigated by the regression analysis method.