This study characterizes the physicochemical properties of coal gangue in Huainan, a typical coal resource-based city in China, and evaluates variations in its chemical composition and associated ecological risks. The results show that the coal gangue in the Huainan mining area is composed mainly of quartz and clay minerals, with SiO₂ and Al₂O₃ together accounting for over 86% of the total composition. Rare earth element concentrations are generally higher than background levels, whereas heavy metal concentrations are generally below the risk screening values for soil contamination of agricultural land. Complex associations are observed among the elements in coal gangue. The correlation coefficients between SiO₂ and the other oxides or heavy metals range from −0.750 to −0.993, indicating significant negative correlations and suggesting that the silicate mineral phase occurs independently of other element-enriched phases. The potential ecological risk index (RI) for heavy metals ranges from 33.75 to 300.71 and is driven primarily by Hg and Cd. The RI for rare earth elements ranges from 98.9 to 220.3, with Lu as the key influencing element. The predicted probability of adverse biological effects is 14–15%. Overall, classified management of coal gangue in the Huainan mining area is recommended, together with strengthened continuous monitoring of Hg and Cd and optimization of ecological disposal strategies by integrating potential ecological risk assessment with analysis of adverse biological effects, thereby further supporting the green transition of resource-depleted cities.