The reductive removal of hexavalent chromium Cr⁶⁺ by iron slag from aqueous solutions was investigated. Iron slag was treated with a grinding miller and washed thoroughly. The redox reaction of iron slag onto the chromium Cr⁶⁺ was initiated by Fenton’s reagent (Fe²⁺/H₂O₂). The optimum conditions for adsorption of Cr⁶⁺ were found to be as follows: pH 3; photocatalyst dose 0.13 g/L; COD 400 mg/L; Cl⁻ concentration 5 mg/L; matrix was gravel and construction waste and contact; reaction time was 270 min. In addition, the action mechanism of each factor in the wastewater was analyzed. In the reaction system, pH was the main influencing factor. When the pH was 3, the reduced rate of Cr⁶⁺ could reach 99%. High salinity had a certain effect on Cr⁶⁺ reduction. When the Cl⁻ concentration was more than 6 mg/L, Cr⁶⁺ reduction rate was below 90%. Owing to high efficiency and low cost, iron slag could be used as an effective catalyst for Cr⁶⁺ removal from wastewater.