This work proposed an in-situ method for stabilizing the hazardous calcium arsenic residues (CARs) by introducing small amounts of ferrous (Fe^II) solution to induce the formation of stable ferrous arsenate minerals. The CARs collected from a copper smelter in China were treated with Fe^II solution at room temperature, Fe^II/As molar ratios of 0.5–2 and pH 6–9 under both oxic and anoxic conditions, and the concentrations of aqueous arsenic (As) was monitored during the whole treatment process (up to 760 days). Results showed that As dissolution was significantly inhibited after addition of Fe^II into the CARs slurries. The concentration of dissolved As was lowered to < 2.0 mg/L from initially ∼ 1000 mg/L at pH 6 and 7, and Fe^II/As ≥ 1. The resultant solids under these conditions showed TCLP As concentrations of < 1.0 mg/L, which were dramatically lower than that of untreated CARs (862 mg/L). Characterization with X-ray powder diffraction (XRD), scanning/transmission electron microscopy (SEM/TEM), and X-ray absorption spectroscopy (XAS) of the resultant solids revealed that the formation of ferrous arsenates minerals (parasymplesite and/or ferrisymplesite) was the mechanism of the stabilization effects. The Fe^II treatment method proposed in this work provides an alternative option for in-situ CARs stabilization.