Abstract: Fly ash, as a bulk industrial solid waste, is widely used for underground backfilling; however, its high alkalinity can readily cause groundwater pH value to exceed permissible limits, posing a potential risk of water quality deterioration. To regulate the alkaline environment of fly ash, highly alkaline fly ash is collected on site from the Huangyanhui Coal Mine solid waste backfilling project for experimental analysis. The study elucidates the mechanisms by which acid modification alters the chemical composition and microstructure of highly alkaline fly ash, and systematically examines the regulatory patterns and sensitivity of pH value with respect to key parameters, including acid type（citric acid, oxalic acid, and hydrochloric acid）, acid concentration（0.1 mol/L, 0.5 mol/L, and 1 mol/L）, and modification duration（8 h, 16 h and 24 h）. The results indicate that acid modification leads to a marked reduction in portlandite content, accompanied by the formation of new calcium-based crystalline phases. Microstructural observations reveal that the spherical morphology of fly ash particles becomes noticeably weakened, while surface attachments increase; acid concentration is identified as the primary factor influencing these microstructural changes, whereas extended modification time exerts a relatively limited effect. Compared with the initial pH value, the pH value reduction of modified fly ash ranges from 5.24% to 65.01%. Range analysis shows that pH value sensitivity to modification parameters follows the order: acid concentration > acid type > modification duration. Increasing the acid concentration from 0.1 mol/L to 1 mol/L results in a pH value reduction of 45.3%, while extending the modification time from 8 to 24 h reduces pH value by only about 4%. The overall pH value reductions achieves by hydrochloric acid, oxalic acid, and citric acid modification is 36%, 33%, and 22%, respectively. Significant differences are observed among the three acid modifiers at the 95% confidence level, with the pH value reduction efficacy ranking as hydrochloric acid > oxalic acid > citric acid. Hydrochloric acid demonstrates the most effective pH value regulation capability, consistent with the range analysis.