The enhancing effect of As(III) oxidation on As stabilization by lime is routinely attributed to the lower solubility of Ca arsenates than Ca arsenites. However, this routine explanation is insufficient for the scenario of As(III) partial oxidation, in which Ca arsenites still predominate As leachability due to the relatively high solubility. In this study, an As–Ca sludge with a high As(III) content (96 g/kg, 55% of the As(tot)) was treated by oxidant–lime to clarify the positive effect of As(III) partial oxidation. Lime alone only reduced As(tot) leaching concentrations from 541 to 4.9 mg/L (4.3 mg/L of As(III) and 0.6 mg/L of As(V)), failing to meet the regulatory limit (2.5 mg/L). After partial oxidation of As(III), lime treatment could further reduce As(III) leaching concentrations from 4.3 to below 1.9 mg/L, whereas As(V) remained stable at about 0.6 mg/L. Qualitative and quantitative analyses based on XRD, SEM–EDS, TG, and thermodynamic modeling suggested that the solubility of newly-formed amorphous Ca arsenites (CaHAsIIIO3•xH2O) after lime treatment determined the final As(III) leachability. The CaHAsIIIO3•xH2O formed at lower As(III) contents due to As(III) partial oxidation had lower solubility products and possibly higher crystallinity, resulting in the lower As(III) leachability. This study provides new insights into the role of As(III) partial oxidation in deep decline of As(III) leachability during lime stabilization, guiding the treatment of As–Ca sludge as well as other As(III)-bearing solid wastes.