The effect of B₂O₃ content on the viscosity of SiO₂-MgO-FeO-based molten slag system was investigated using the rotating cylinder method. The evolution process of the melt structure under different contents of B₂O₃ was comprehensively studied via FTIR spectroscopy and a model for calculating the degree of polymerization was developed. The results showed that the viscosity of the molten slag decreased with the addition of B₂O₃, which had a slight effect when its content exceeded 3 wt.%. As the addition of B₂O₃ increased from 0 to 4 wt.%, the break temperature of the slags decreased from 1152 to 1050 °C and the apparent activation energy decreased from 157.90 to 141.84 kJ/mol. The addition of B₂O₃ to the molten slag destroyed the chain silicate structure to form a more cyclic borosilicate structure. The Urbain model was improved to calculate the viscosity of the SiO₂-MgO-FeO-based slags, and the values were in good agreement with the experimentally measured values.