The viscous behavior of CaO–SiO2–15.2 wt% Al2O3–4.8 wt% MgO–TiO2–B2O3 slags is studied. The effect of the TiO2 and B2O3 on the viscosity and apparent activation energy, as well as the effect of B2O3 on structure is investigated through Raman spectroscopy. The viscosity and apparent activation energy of CaO–SiO2–15.2 wt% Al2O3–4.8 wt% MgO–TiO2 are decreased with increased TiO2 content, which can be explained by the network modifier role that TiO2 acts in slags. The addition of B2O3 can reduce the viscosity of CaO–SiO2–15.2 wt% Al2O3–4.8 wt% MgO–TiO2–B2O3 slags, and it brings about a turning point on apparent activation energy curves. Raman spectroscopy shows that B2O3 acts as acidic oxide and increases the degree of polymerization of slags. However, with the addition of B2O3, the appearance of two dimensional [BO3] structure units and the low strength energy of B–O bonds can reduce the viscous resistance in molten slags and decrease the viscosity.