Abstract: The valence state and mobility of arsenic (As) are closely related to redox potential (E_H). Changes in E_H may lead to an increasing mobility risk of As in the stabilized/solidified soils when exposed to complex redox environments. At present, the influences of E_H on the leaching behaviors of As from the solidified soils have received few attention. In this study, the semi-dynamic leaching tests under different E_H are carried out to investigate the leaching behaviors of As from a stabilized/solidified heavily As(Ⅲ)-contaminated soil. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to study the changes of mineral composition and the valence state of As. The results show that the total leached As in the leachate is closely related to E_H of the leachant. The leached As concentration increases with a decrease in E_H. When E_H of the leachant is 0 mV, the diffusion coefficient of As reaches 3.11×10⁻¹³ m²/s, and its leachability index reaches 8.72, indicating that the treated heavily As(Ⅲ)-contaminated soil is not suitable for stacking and reuse under a strong reducing condition. The XRD analysis shows that as the leaching E_H decreases, the reductive dissolution of iron oxides/hydroxides occurrs, which leads to the desorption of As. The XPS investigations indicate that as the leaching E_H decreases, As(Ⅴ) in the soil is reduced to highly mobile As(Ⅲ), and the potential mobility risk of As increases. This study provides a scientific basis for the long-term safe disposal of heavily As(Ⅲ)-contaminated soils.