The structure-viscosity relationship of the low-silica (SiO₂ ≤ 10 wt%) calcium aluminosilicate melts, which represent the secondary refining ladle slag systems, was investigated by employing the rotating-cylinder viscosity measurement in conjunction with the Raman spectroscopy measurement for linking the macroscopic thermophysical property and molecular (ionic) structural information. Furthermore, the influence of CaF₂ on the structure-property relationship was explored. The viscosity of low-silica calcium aluminosilicate melts decreased with increasing both CaO/Al₂O₃ and CaO/SiO₂ ratios. However, the effect of the former on the viscosity of low-silica calcium aluminosilicate melts was larger than that of the latter. By employing the Neuville’s structure model, in which the silicate structural units with various NBO, i.e. Qⁿ_Si are located at the boundary of the AlO₄ aluminate, and the Raman scattering data of the glass samples, it was demonstrated that the aluminate and silicate units are more effectively modified by increasing the CaO/Al₂O₃ ratio at fixed silica content. The addition of small amounts of CaF₂ (~5 wt%) to the low-silica calcium aluminosilicate melts decreased the viscosity of the melts. From the analysis of Raman scattering data, the liberation of SiO₄^4– (Q⁰_Si) units from the AlO₄ aluminate structure by addition of CaF₂ was understood. However, the effect of CaF₂ addition on the viscosity became less discernible at higher CaF₂ content (≥ 10 wt%) region, where the F ions simply substitute for the non-bridging oxygen ions in AlO₄ tetrahedra.