Abstract: Mitigating the environmental impact of bulk industrial solid waste from yellow phosphorus smelting requires efficient and collaborative resource utilization of by-products. This study systematically investigates the pozzolanic activity activation mechanism of yellow phosphorus slag (YPS) and evaluates its performance as a primary component in underground cemented mine backfill systems. The physicochemical properties of YPS and phosphogypsum (PG) were characterized, revealing that YPS was rich in CaO (50.66%) and SiO₂ (37.33%), which provided a solid chemical foundation for the formation of C–S–H and AFt hydration products under alkaline conditions. Extensive mix proportion experiments were conducted on YPS-based supplementary cementitious materials to determine the optimal formulation that balanced early age hydration reactivity with long-term economic feasibility. A novel “chemical–mineral” composite excitation strategy utilizing calcium oxide (CaO), coal fly ash (FA), and sodium sulfate (Na₂SO₄) was employed to overcome the inherent low early-stage reactivity of YPS. The optimized binder matrix formed the basis of an innovative all-phosphorus-based solid-waste-cemented backfill system. This study examined the synergistic effects of varying the substitution rate of YPS-based cementitious material and YPS structural aggregates on the macroscopic unconfined compressive strength of the backfill body. To provide robust theoretical validation for the observed macroscopic mechanical behaviors, advanced microscopic characterization techniques, including thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM), were employed. The experimental results demonstrated that the implementation of an "alkali–sulfate" composite excitation strategy can effectively activate the latent cementitious properties of YPS. Specifically, using an excitation composite consisting of 20% CaO, 26.7% FA, and 5% Na₂SO₄, the newly developed YPS-based cementitious material achieved a remarkable maximum 28-day compressive strength of 13.15 MPa. When this optimized binder formulation was applied to the YPS–PG composite backfill system, the macroscopic strength decreased as the substitution rate of the YPS cementitious material increased. However, replacing fine PG with coarse YPS as an aggregate resulted in a pronounced enhancement in the strength. Under a relatively rich binder condition (sand-to-binder ratio of 2), the compressive strength exhibited a distinct evolutionary trend of “an initial increase followed by a subsequent decrease” as the YPS aggregate substitution rate increased, reaching an optimal peak at a 25% substitution rate due to the ideal “skeleton-filling” packing density. Conversely, under lean binder conditions (sand-to-binder ratio of 3), the effect of the aggregate substitution rate on the mechanical performance demonstrated a monotonic “continuous improvement” characteristic, highlighting the critical structural support provided by the coarse YPS particles. Ultimately, this study proves that the dual-substitution strategy can effectively mitigate the strength degradation typically caused by a single binder substitution. This approach not only maintains the stringent mechanical stability required for underground backfilling (achieving 3.45 MPa with the optimal mix) but also significantly reduces the overall raw material costs by 22%–23%. The findings demonstrate significant synergistic optimization effects, balancing exceptional mechanical performance with economic benefits and offering a highly viable technological pathway for the large-scale disposal of multi-source phosphorus-based solid wastes in the circular economy of green mining.