Abstract: Considering that the viscosity of molten slag cannot be accurately calculated from(NBO/T) ratio using the traditional method, this paper firstly studied the effects of different oxides on the polymerization degree of molten slag, proposed a new method for calculating(NBO/T) ratio, and developed an estimation model of viscosity for aluminosilicate melts based on(NBO/T) ratio.The new viscosity model was adopted to predict the viscosities of the ternary system of CaO-SiO₂-Al₂O₃ and the quaternary systems of CaO-SiO₂-MgO-Al₂O₃ and CaO-SiO₂-Al₂O₃-RO(R₂O), and the calculated results were satisfactory in comparison with those of the Urbain model and the modified Iida model.The present model can predict the viscosity of the CaO-SiO₂-MgO-Al₂O₃-R₂O(K₂O or Na₂O) system effective.