Abstract: To address the dual challenges of resource utilization of mining solid waste (e.g., coal gangue) and performance enhancement of cemented rockfill, this study systematically investigates the mechanisms of ultrasonic dispersion time and polycarboxylate superplasticizer (PCE) on the properties of cellulose nanofiber (CNF)-modified cemented rockfill. A series of comparative experiments were designed with varying ultrasonic dispersion times (0–60 min) and PCE dosages (0.1wt%–0.4wt%). Through mechanical testing, hydration product analysis, and microstructural characterization, the study revealed the advantages of PCE in promoting CNF dispersion to enhance the engineering applicability of cemented rockfill. The results demonstrate that: (1) Ultrasonic dispersion for 30 min increases the compressive strength by 37.7% compared to the untreated group; however, excessive ultrasonication (60 min) induces hydrolysis of CNF, releasing reducing sugars that retard hydration. (2) PCE facilitates CNF dispersion, achieving a 29.1% increase in compressive strength at a dosage of 0.4wt%, while simultaneously improving hydration products and microstructural development. (3) While ultrasonic dispersion yields slightly higher strength improvements, PCE demonstrates superior cost-effectiveness and operational convenience, rendering it more viable for industrial adoption. This study provides a theoretical foundation for the nano-enhanced modification of cemented rockfill, offering new insights into the recycling of solid waste and the development of high-performance materials.