This study takes the Southern antimony smelting slag depot in Xikuangshan (XKS) Sb mine in the city of Lengshuijiang, Hunan, China, as the research object and explores the release law of Sb, As, and Hg in smelting slag under different pH-value simulated acid rain by a semi-dynamic leaching experiment of simulating the local rain. The results show that the leachate pH value is positively correlated with the pH value of simulated acid rain, while the leachate conductivity is negatively correlated with it. The leaching rates of As and Hg are negatively correlated with the pH value of the simulated acid rain, while the leaching rate of Sb is positively correlated with it; in the leaching process, the leachate pH value goes downward slowly after shooting up, but the leachate conductivity is continuously reduced; the releasing process of Sb, As, and Hg consists of two stages, and their leaching rate forms such an order as Sb > Hg > As. The diffraction peak intensity of the main mineral composition of quartz and calcite decreases significantly after leaching; the smelting slag’s surface becomes less rough than before leaching, with fewer pores and edges, and the contents of S, Si, Al, Fe, Ca, and Sb on the slag surface decrease while the content of O, As, and Hg increases.