In the flotation of semi-soluble minerals such as calcite and fluorite, Ca²⁺ is one of the common dissolved components influencing the collector adsorption behaviors on the mineral surfaces. However, there is very limited research focusing on how the dissolved Ca²⁺ affects the separation of fluorite and calcite. In the study, with sodium oleate (NaOL) as a collector and tannic acid (TA) as a depressant, a flotation test, zeta potential measurement, and adsorption experiment in the presence of Ca²⁺ were conducted. Flotation tests indicated that in the presence of Ca²⁺, fluorite and calcite were both significantly depressed by TA, leading to difficulty in the separation of the two minerals. It was also found that the depression effect on fluorite flotation was minimized with a high concentration of NaOL. Zeta potential measurement and the adsorption experiment results are consistent with the flotation results, revealing that it is only in the low concentration of NaOL that the flotation of fluorite was depressed by TA in the presence of Ca²⁺. Specifically, in the low concentration of NaOL, TA adsorbed on the fluorite surface and depressed the flotation of fluorite due to the preferential interaction between NaOL and Ca²⁺ in the solution leading to a shortage in the effective NaOL in the solution. In the high concentration of NaOL, the adsorbed TA on the fluorite surface was displaced by the excessive NaOL in the solution; hence, the flotation of fluorite was recovered. In contrast, TA always hinders the interaction of NaOL with calcite regardless of the presence and absence of Ca²⁺ and NaOL, hence, depressing the flotation of calcite. The study presented that a high concentration of NaOL may mitigate the negative effect of Ca²⁺ on the fluorite surface and improve the separation of fluorite from calcite.