This study focused on the ion release and microstructure of slag during its degradation following erosion by different pH solutions. It focused on controlling factors such as slag particle size range, pH value of the solution, and soaking time. The surface microstructure and particle size distribution of slag with the particle size of 0.075–5.0 mm, the mineral composition of suspended pollutants larger than 0.45 μm, and the phenomenon of nano-scale ion release were examined. When slag was soaked in solutions with different pH values for 30 days, the pH value of leachate tended to be neutral, the release amount of Ca, Mg, Fe, and Cd ions increased and the release rate gradually decreased. The dissolution process of slag in the alkaline solution was slower than that in acid, but suspension and gels formed more easily in an alkaline environment. Nitric acid accelerated the chemical reaction of akermanite, gehlenite, and hawleyite, and released Ca, Mg, and Cd ions. There were clear damage cracks and various irregular pores on the slag surface. Under the attack of alkali solution, the weight of akermanite in slag increased, the Mg ion content in solution decreased, and the suspended solids of calcite and portlandite increased. At pH 12, unlike at pH 3, there were no large surface cracks in the slag and the interface damage was small. Compared with pH 7, there were more irregular substances, such as flakes and spheres. The particle size of slag was mainly 0.1–0.5 mm, the content before and after leaching was 52.80%–55.87% and 55.00%–58.27%, and the slag was in a poor grading state. The findings of this study act as an important reference for understanding the influence of slag leaching on water and soil pollution.