Abstract: In the pure mineral system, carboxymethyl cellulose (CMC) exerts a stronger depressing effect on calcite than on scheelite; however, it loses its selectivity significantly in the their mixed mineral system, and the microscopic action mechanism underlying this phenomenon remains unclear. For this reason, the microscopic mechanism responsible for the failure of CMC’s selective depression in the scheelite−calcite mixed mineral system was investigated via flotation experimental design, solution chemistry calculations and surface characterization techniques.These complementary methods ensure the accuracy and reliability of our experimental data and findings. The results indicate that the addition of calcite either before or after CMC significantly enhances the depressive effect of CMC on scheelite, thereby depriving CMC of its selective depression ability towards both scheelite and calcite.This is mainly because the calcium ions released from the dissolved calcite enter the solution system, whereby a small fraction adsorbs onto the scheelite surface to provide active sites for CMC while the majority exist in a free state in the solution and undergo a chemical reaction with CMC to form Ca−CMC complexes, thereby increasing the adsorption density of CMC on the scheelite surface. This action mechanism is independent of the addition sequence of calcite (or calcium ions) and CMC, and thus the depressing effect of CMC on scheelite is significantly enhanced irrespective of the addition order. In addition, the depressing effect of CMC on scheelite flotati disappears correspondingly when calcite dissolution is inhibited by the common ion effect of sodium carbonate, which further confirms that the solution chemical reaction between calcium ions and CMC induced by calcite dissolution is the microscopic pathway for enhancing the depression of scheelite flotation by CMC. This study deepens the understanding of the interaction mechanism between scheelite and calcite, and also demonstrates that regulating the solution chemical environment can effectively modulate their interaction, thereby providing a theoretical basis for the research on mineral interactions in flotation.