Abstract: Admixtures are widely used to improve the filling performance of full−tailings slurry, but there is relatively little research on the mutual influence between different types of admixtures. To investigate the synergism and mutual site−occupancy effect of the admixtures on full−tailings slurry, the water reducing agent of polycarboxylate ether (PCE) and the thickening agent of hydroxypropyl methylcellulose (HPMC) were mixed into the molybdenum full−tailings slurry. The rheological parameters of the full−tailings slurry with different admixture dosages were obtained through rheological performance testing, and the influence of the blending of the additives on the rheological properties of the full−tailings slurry was analyzed. The carbon adsorption measurement method was employed to determine the adsorption capacity of the additives and to explore the synergism and site−occupancy mechanism between the additives in the full−tailings slurry. The results indicate that the combination of PCE (0.03%) and HPMC (0.10%) has a higher plastic viscosity and a lower yield stress, which is considered to be optimal. The synergism between the additives can be fully produced by reasonably controlling the dosage of thickeners and water reducers in the full−tailings slurry, ensuring the maximum adsorption capacity of 1.12 mg/g with minimal dosage of additives, which can not only improve the fluidity of the full−tailings slurry but also enhance its stability, thus achieving the maximization of strengths and avoidance of weaknesses. However, when the dosage of the two individual additives is excessive, the total adsorption capacity of the additives on the unclassified tailings is less than the sum of the adsorption capacity of the two single additives. This is attributed to the site−occupancy effect between HPMC and PCE, which destroys the adsorption capacity of the particles on the additives. Therefore, it is crucial to control the dosage of the different types of additives in a sensible way, so as to create a synergism between the additives that can improve the performance of the filling slurry.