The performance of stabilization/solidification(S/S) monoliths is vital for the long-term effectiveness of potentially toxic-element-contaminated soil treatments, which arevulnerable to environmental conditions (e.g., strongly acidic or alkaline conditions). This study numerically investigated the long-term performance of S/S monolith materials in erosive environments with pHs ranging from 2.65 to 10.00 over three years. It was found that the leachability and availability of Pb decreased continually over the three years due to the transformation of the chemical fractions of Pb from acid-soluble fractions to reducible and oxidizable fractions. Environmental pH greatly affected the strength and permeability of the S/S monoliths, which increased in weakly acidic and alkaline conditions (e.g., pH 5.00–10.00), while they were significantly reduced in strongly acidic conditions (e.g., pH 2.65–3.65). The strength of the S/S monolith decreased from 19.7 MPa to 0.2 Mpa after three years when the environmental pH was 2.65. A model was established to predict the long-term strength evolution of the S/S monolith. The dissolution of hydration products in the S/S monolith materials caused by acid erosion directly attenuated the strength and permeability of the S/S-treated soils. Long-term exposure to acidic environments could lead to the failure of S/S treatments. Thus, more attention should be paid to the long-term effectiveness of S/S monoliths in erosive environments.