Abstract: High-volume fly ash (HVFA) binders are widely utilized as a mature method for cemented paste backfill in green mining, yet their performance remains highly sensitive to mix design. The fundamental coupling mechanism between the water-to-binder mass ratio (W/B) and sodium lignosulfonate (SL) content in pozzolan-rich HVFA systems remains insufficiently understood. In this study, HVFA pastes with varying SL contents (0–0.9wt%) and W/B ratios (0.5–0.8) were characterized via rheometry, unconfined compressive strength (UCS) tests, and microstructural analyses, including zeta potential measurements. Results indicated that the absolute zeta potential magnitude increased from 11.88 to 27.08 mV as SL dosage rose from 0 to 0.9wt%, providing direct evidence for enhanced electrostatic repulsion. This surface modification significantly reduced yield stress and decreased the Relative Thixotropic Index (RTI) from 14.99% to 7.88% at a W/B of 0.5 with 0.3wt% SL. The effect of SL on 28-d UCS was non-monotonic, peaking at 35.72 MPa with 0.3wt% SL. The mercury intrusion porosimetry (MIP) analysis revealed a primary pore diameter shift from the harmful range (~284 nm) to the refined range (183 nm), while X-ray diffraction (XRD) analysis confirmed enhanced calcium hydroxide consumption via pozzolanic reactions. The findings elucidate the dual role of SL as a physical dispersant optimizing particle packing and a chemical modulator governing hydration kinetics. These quantitative relationships provide a scientific basis for the performance-based design and intelligent pumping control of HVFA binders.