Molecular dynamics (MD) simulations have been used to study the structure properties of CaO–SiO2 slags and CaO–SiO2–B2O3 slags with different basicity. The results show that the addition of B2O3 to the higher basicity system of CaO–SiO2 (basicity >1.12) increases the degree of polymerization (DOP) of the system and the complexity of network structures, theoretically leading an increase in the viscosity of the slag. In the CaO–SiO2–B2O3 ternary system, the structure of Si–BO (bridging oxygen)–B is more dominant than B–BO–B, indicating that the newly introduced B–O network structure will be randomly mixed with the initial Si–O network structure instead of forming enrichment zones. Moreover, with the addition of B2O3, the viscosities of CaO–SiO2 increase in higher basicity systems, but decrease in the lower basicity systems based on the viscosity measurement. The different effect of adding B2O3 on the viscosity of the CaO–SiO2 slag is consistent with the changing trend of the melt structure.