[Purposes] This study uses coal gangue as subgrade fill material and explores the release-migration-solidification characteristics of heavy metal pollutants under the influence of natural rainfall. [Methods] Laboratory experiments were conducted to determine the basic properties of coal gangue fill material, identify the types of heavy metal pollutants, and investigate the release-migration characteristics of heavy metal pollutants during natural rainfall. Based on porous media solute transport theory, a numerical model was established to predict the impact of pollutants on the soil-water environment under extended rainfall. The solidification effects of geopolymers on pollutants in coal gangue fill material were examined at different mass fraction levels and curing ages. The effectiveness of geopolymers in chelating heavy metal pollutants in coal gangue fill material was explored. The solidification characteristics of geopolymer-solidified coal gangue fill material were comprehensively evaluated by comparing them against the cement solidification method. [Findings] The leaching mass concentrations of Pb and As heavy metal pollutants in coal gangue exceeded environmental limits, which were 9.1 times and 9.8 times greater than the Class Ⅲ groundwater limit in Standard for Groundwater Quality, respectively. In the early stages of rainfall, various heavy metal pollutants in coal gangue fill material exhibited rapid release rates and substantial leaching, followed by a gradual decrease in release rate until reaching a plateau, with cumulative release amount continuously increasing. Heavy metal pollutants migrated towards the bottom of the subgrade over time. After adding geopolymers, the leaching concentrations of all heavy metals were much lower than the required limit in the environmental standards. [Conclusions] Heavy metal pollutants in coal gangue as subgrade fill material exhibited persistent and long-term environmental impact. With increasing geopolymer content and curing age, the mass concentrations of heavy metals in the leachate significantly reduced and were much lower than the Class Ⅲ water limits. Compared to cement solidification, geopolymers demonstrated a superior solidification effect. The optimal solidification treatment of pollutants was recommended as the addition of 8% geopolymer with 14 curing days.