The viscous behavior of CaO–SiO2–MgO–TiO2–Al2O3–B2O3 blast furnace (BF) slag system was studied. The effects of the B2O3 and the TiO2 on the viscosity and break point temperature, as well as the correlations of the properties with the slag structure were analyzed. According to the experiment results, an increase in the TiO2 content tended to enhance the slag viscosity and break point temperature of the slag due to the formation of perovskite, whereas B2O3 had an opposite effect on them. In addition, increasing B2O3 content seems to be more effective in affecting the viscosity compared to TiO2 additions. Depolymerization of the slag after adding B2O3 should be attributed to decrease in the quantities of perovskite.