Abstract: Tungsten, a strategic non-ferrous metal critical for advanced industrial applications, predominantly exists as underutilized scheelite resources characterized by fine-grained intergrowths with calcite that are challenging to separate. This study deciphers the atomic-scale mechanism underlying the selective flotation separation of scheelite from calcite mediated by hydrolyzed polymaleic anhydride (HPMA), a novel environmentally benign reagent, through integrated experimental characterization and computational simulations. Micro-flotation assays quantitatively demonstrated HPMA’s exceptional selectivity, suppressing calcite recovery from 91.64% to 18.61% at pH 7 (10 mg·L⁻¹ dosage) while preserving scheelite floatability. Fourier transform infrared spectroscopy revealed HPMA preferentially adsorbs on calcite, efficiently hindering sodium oleate (NaOL) attachment, whereas NaOL selectively binds to scheelite. X-ray photoelectron spectroscopy analysis confirmed carboxyl (–COO⁻) group chemisorption at calcite’s Ca sites, evidenced by a 0.26 eV negative shift in Ca 2p_3/2 binding energy and new Ca–O bond formation. Density functional theory (DFT) simulations quantified adsorption energetics: HPMA exhibited stronger affinity for calcite (104) surfaces (−1166.441 kJ·mol⁻¹) versus scheelite (112) (335.180 kJ·mol⁻¹). Mulliken bond population analysis quantified interfacial bonding nature. The calcite–HPMA interface formed polar covalent bonds (populations 0.23–0.28), contrasting with NaOL’s ionic interactions (population 0.13) on scheelite. This covalent advantage enables HPMA to preferentially passivate calcite surfaces, suppressing NaOL co-adsorption and facilitating selective scheelite recovery through differential surface reactivity modulation.