In this study, the structure, viscosity characteristics, and crystallization behavior of CaO-SiO₂-B₂O₃ based melts were studied combining molecular dynamics (MD) simulation, Fourier transform infrared (FTIR) spectroscopy, rotating viscometer test, and FactSage thermodynamic calculation. The results showed that, in the ternary CaO-SiO₂-B₂O₃ glass system, stable structural units of [SiO₄]⁴⁻ tetrahedral, [BO₃]³⁻ trihedral and [BO₄]⁵⁻ tetrahedral were formed, and the Si-O and B-O structure depolymerize with the basicity increase from 1.15 to 1.25, then the B-O structure become complex with the basicity further increase to 1.35. In fluorine-free mold fluxes, with the basicity increases, the viscosity at 1300 °C increases, the liquidus temperature decreases and then increases, the network structure polymerizes, it indicates that the structural complexity rather than the melting property change plays a predominant role in increasing the viscosity at 1300 °C. Moreover, due to the changes in crystallization phase and solid solution ratio, the viscosity-temperature curve of fluorine-free slag shows the characteristics of alkaline slag and the break temperature increase with the basicity increase. The MD simulation, FTIR experiment, viscosity test, and FactSage calculation results are verified and complemented each other.