<p>Heavy metal contaminated expansive soil poses serious risks to engineering characteristics and external environment due to the flow of pollutants with groundwater and its significant cracking behavior. The contaminated expansive soil is treated effectively using lime to inhibit crack development and contaminant mobility, which will be deteriorated after sustained wetting–drying cycles. To reveal the deterioration mechanism, mineral composition and microstructural, cracking, and leaching characteristics were experimentally investigated. The results showed that the mineral components remaining after the cyclic wetting–drying process retarded the deterioration of the treated soil. Different mineral characteristics and treated effects were attributed to mineral disintegration and transformation during the entire wetting–drying process. Compacted pores and filled cracks were present during the initial wetting–drying cycles, whereas microcracks preferentially appeared and developed as the wetting–drying progressed. The evolution of the total crack length and average crack width with wetting–drying cycles were divided into three stages, where the important role of crack width to determine the cracking characteristics was confirmed. Crack development was verified to control the leaching characteristics and further the deterioration mechanism of the treated soil experiencing wetting–drying cycles. A linear relationship between the cumulative fraction of leached Pb²⁺ and the gray-level entropy was established for quantitative prediction.</p>