Abstract: To address the deterioration of rheological properties caused by the competitive adsorption of coal gangue powder and cement on polycarboxylate superplasticizers (PCEs) in coal mine backfill slurries, this study systematically investigated the adsorption behavior and dispersion mechanisms of two types of superplasticizers—conventional comb-shaped (PCE-A) and short-main-chain long-side-chain (PCE-B)—in coal gangue-cement systems. Multi-scale characterization techniques, including scanning electron microscopy (SEM), Zeta potential analysis, and adsorption-rheology coupling tests, were employed to analyze the morphological features, mineral composition of coal gangue powder, and physical parameters of cement-gangue slurries. The results indicate that the high specific surface area (332 m2/kg) and clay-rich composition of coal gangue powder significantly degrade slurry fluidity. PCE-B mitigates viscosity growth by suppressing clay interlayer intercalation via its short main chain and enhancing steric hindrance through long side chains. Langmuir isotherm analysis reveals that the saturated adsorption capacity of PCE-B on gangue powder (1.04 mg/g) is 42% lower than that of PCE-A (1.79 mg/g). The molecular design of PCE-B, balancing competitive adsorption and optimizing spatial repulsion, significantly improves the pumpability of backfill slurries with high gangue powder content (>30%).