The effect of CaO/Al2O3 mass ratio (C/A) and fluorine content on the viscosity and structure of CaO-Al2O3-based mold fluxes has been researched in this paper. The viscosity results indicated that increasing fluorine only slightly decreases the viscosity of the slag melt, and higher C/A is also observed to decrease the viscosity of molten slag when the C/A changes from 1.3 to 1.7. Structural analysis of the as-quenched fluxes using the Raman spectroscopy showed that the amounts of Al-O-0 and Si-O-Al structural units all decrease with higher fluorine content and C/A, indicating that a depolymerization of the molten structure is occurring. The results of Al-27 and F-19 magic angle spinning nuclear magnetic resonance showed that fluorine tends to participate in the network structure and coordinate with Al3+ ions to form complex ionic clusters. The results suggested that the role of fluorine in the CaO-Al2O3-based slag system is different from the traditional slag system in which fluorine only acts as a diluent, thus reducing the effect of fluorine on lowering the viscosity. In addition, the coordination environment of Al3+ ions can be simplified by higher C/A through promoting the generation of [AlO4](-) tetrahedral structures. Besides, the free O2- ions provided by excess CaO would break the Al-O-0 bonds and further depolymerize the network structure, thereby decrease the viscosity.