Abstract: The Bayan Obo ore contains relatively high contents of fluorine, potassium, and sodium, which cause K₂O, Na₂O, and CaF₂ to exert significant influences on the metallurgical properties of primary slag formed during the blast furnace smelting of this ore. However, the influence mechanisms of these components on the primary slag properties remain insufficiently understood. This work systematically investigated the effects of w（K₂O）/w（Na₂O） and w（CaF₂） on the viscosity and structure of CaO-SiO₂-MgO-Al₂O₃-FeO based blast furnace primary slags. The viscosity of the blast furnace primary slag was determined by the rotating cylinder method, and the microstructural characteristics of the slag were investigated using Raman spectra. The viscosity experimental results indicate that the addition of Na₂O alone decreases the viscosity of the primary slag, whereas an equivalent mass substitution of K₂O for Na₂O increases the viscosity. Moreover, the viscosity of the primary slag gradually decreases with increasing w（CaF₂）. Raman spectra analyses reveal that the equivalent mass substitution of K₂O for Na₂O reduces the number of O^2- ions and inhibits the conversion of O⁰ to O^-, thereby promoting the polymerization of the slag network. The Al—O bonds in AlO₄ tetrahedra charge-compensated by K⁺ ions are stronger than those charge-compensated by Na⁺ or Ca²⁺ ions, leading to the formation of more stable aluminate structural units, which ultimately results in the viscosity increase of the primary slag upon the equivalent mass substitution of K₂O for Na₂O. With increasing CaF₂ contents in slags, the F^- ions provided by CaF₂ promotes the depolymerization of the silicate network structural units, thereby simplifying the slag structure and reducing the viscosity. Furthermore, as a flux, CaF₂ significantly reduces the liquidus temperature of the slag and therefore enhancing its flowability under the experimental conditions. As a result, the viscosity of the primary slag decreases with w（CaF₂） increasing. The findings of this study provide a theoretical foundation for optimizing the performance of primary slag in the blast furnace smelting of Bayan Obo ore.