<p>Fluorine and cerium easily form fluorine–cerium complex ions in extraction fluids, which change the separation coefficient between neighboring rare-earth elements and are very disadvantageous to rare-earth extraction and separation. On the basis of the traditional extraction process, to further elucidate the effect of fluorine–cerium complex ions on extraction performance, we performed density functional theory calculations and used the continuous variation and mole ratio methods to analyze the relevant structures and complexation behaviors of fluorine–cerium complex ions. The results showed that the configuration energy of [CeF₂]²⁺ was the lowest and relatively stable. Complexation experiments involving the continuous variation and mole ratio methods verified that the F⁻-to-Ce⁴⁺ complexation ratio was approximately 2. The complexation reaction was carried out in the presence of CeF₂²⁺ with a complexation equilibrium constant <i>β</i> of 5.1658 × 10¹³. The complexation reaction of F⁻ with Ce⁴⁺ reached equilibrium in 2&#xa0;h. The thermodynamic properties of this fluorine–cerium complexation reaction were investigated, and Δ<i>H</i> &gt; 0, Δ<i>G</i> &lt; 0, and Δ<i>S</i> &gt; 0 indicated that the complexation process was a spontaneous heat-absorption process. The experimental results provide theoretical guidance for the efficient extraction and separation of rare-earth elements and the efficient utilization of resources.</p>