The mechanical properties of hemihydrate phosphogypsum-based backfill (HPG-backfill) are significantly influenced by the temperature and pH of mine water (MW), impacting the stability of underground mining operations. This study evaluates the effects of MW at different temperatures (20 degrees C, 30 degrees C, and 40 degrees C) and pH levels (3, 5, and 7) on HPG-backfill's mechanical strength. A comprehensive analysis, including uniaxial compressive strength (UCS) testing, scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric and differential thermogravimetric (TG-DTG), and nuclear magnetic resonance (NMR), was employed to explore degradation mechanisms. The results indicate a significant decline in the mechanical performance of HPG-backfill when exposed to MW. This degradation becomes particularly pronounced under more acidic conditions and at elevated temperatures. A polynomial relationship between strength and pH, and a linear correlation with temperature, were identified. Interaction effects between temperature and pH on 28-day strength degradation were observed, diminishing with increased temperature or decreased pH. Gray relational analysis highlights pH as a more critical factor than temperature in degradation. Strength degradation is primarily attributed to gypsum dissolution and the pressure induced by recrystallization, which leads to the formation of fatigue cracks. Additionally, acidic conditions accelerate premature crystallization, altering both the crystal morphology and the pore structure. These insights advance the understanding of HPG-backfill degradation, guiding the developing of more resilient backfill materials for extreme mining environments.