The CaCl$_2$-CaF$_2$-CaO phase diagram was investigated in the CaO low region (<40 mol.%). CaCl$_2$-CaF$_2$ and CaCl$_2$-CaO binary diagrams, constituting the ternary system, were first studied by Differential Scanning Calorimetry (DSC) measurements and X-Ray Diffraction (XRD) characterization; a good agreement was obtained between the phase diagram models calculated with FactSage® software (FTsalt database) and present experimental data. As the CaF$_2$-CaO liquidus could not be measured by DSC due to the high melting temperature, this diagram was calculated using FTsalt database combined with FToxid database of FactSage® software. The ternary phase diagram was obtained by calculations and exhibits an eutectic at the composition CaCl$_2$-CaF$_2$-CaO (78.2-15.7-6.1 mol.%) melting at 637˚C, and five peritectic points. Measurements of relevant vertical cross-sections for three CaCl$_2$-CaF$_2$ compositions (50-50, 40-60 and 30-70 mol.%) up to 18 mol.% CaO are in agreement with the ternary phase diagram model. For each section, the liquidus temperature is constant up to around 11 mol.% CaO and then sharply increases. Moreover, an increase of CaF2 content in CaCl$_2$-CaF$_2$ melt leads to a decrease of the CaO solubility in isothermal condition.