The viscosity of CaO-Al2O3-SiO2-TiO2-MgO-Na2O system slag was measured at 1773 K under the molten reduction condition by the restricted mixture uniform design method. The regression equation between slag composition and viscosity was established using the partial least squares regression analysis. The effects of basicity (CaO/SiO2), MgO, TiO2, Al2O3 and Na2O on the viscosity of slag were analyzed by the regression equation with relevant phase diagrams, literature and XRD phase analysis method. The results indicate that the correlation coefficient RC2 fitting to the regression model is 0.9945, indicating good agreement of the experimental data with the regression model, which can predict well the viscosity of slag. In the experimental composition ranges, the viscosity increase with increasing of basicity [w(CaO)/w(SiO2)]. When the basicity is fixed, the viscosity decreases with increasing of the contents of MgO, Al2O3 and Na2O. With increasing of TiO2 content, the viscosity first increases and then decreases. When the basicity of slag is less than 3.1, the mass fraction of MgO more than 4%, that of Al2O3 more than 20%, that of TiO2 in 3.1%~6.1%, that of Na2O more than 0.75%, the viscosity of slag is less than 2Pa×s at 1773 K, which fully meets the requirements of actual industrial operation.