The study of armoring effect is helpful to understanding the mechanism of acid mine drainage (AMD) releasing from limestone-rich metal sulfide tailings (limestone tailings). By performing static experiments of AMD immersing limestone particle materials, the authors studied the formation of secondary armor under acid water saturation condition, similar to the circumstances in acidified tailings. The results show that Fe content in AMD may be one of the main factors affecting the armoring effect of limestone. In high Fe concentration AMD (1029~1033 mg/L), the order of secondary precipitants is Al-rich phase→gypsum→lepidocrocite (adsorbing Zn, Cu and As) on the surface of limestone particles, and the precipitants make the limestone armored/passivated, causing the solution to maintain acidity. By contrast, in low Fe concentration AMD (6.71~74.8 mg/L), the secondary armor may not form, therefore limestone can be dissolved and release a large amount of CO₃^2-, which fully neutralizes H⁺ in solution, so that the solution can reach neutral circumstances. Based on this study, the authors hold that the limestone tailings with high Fe sulfide content (e.g. Fe 10.62%, S 5.70%) could have a higher risk of releasing AMD (and heavy metals), because the tailings could produce the acid water with high Fe content through oxidation and the armoring effect of limestone could occur easily.