This study systematically compared the depressive behaviors and mechanisms of carboxymethyl cellulose (CMC) towards two minerals through single mineral flotation tests, artificial mixed ore flotation tests, combined with contact angle measurements, zeta potential analysis, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) characterization. The results demonstrated that CMC exhibited significant depression on ankerite while showing minimal inhibitory effect on columbite. Under optimized conditions (pH=8.0, CMC dosage 7.5 mg/L, BHA concentration 1.0×10_^-2 mol/L), the artificial mixed ore flotation achieved high-quality concentrate with a Nb₂O₅ grade of 69.45% and recovery rate of 81.15. Mechanistic investigations revealed that CMC only underwent weak physical adsorption on columbite surfaces, whereas chemically adsorbed onto calcium (Ca) active sites of ankerite. This selective adsorption behavior not only directly suppressed ankerite flotation but also prevented subsequent BHA adsorption by occupying surface active sites, ultimately enabling efficient separation of the two minerals. The adsorption mechanism of CMC was further verified through first-principles calculations.