In this study, the effect of initial dissolved oxygen (DOi) on the efficiency of removing Cd2+ from wastewater using a newly designed closed-atmosphere electrocoagulation (CAEC) system was investigated. The effects of factors such as temperature, initial pH, current density, and electrolyte composition (NaCl and Na2SO4) were investigated. The results indicated that the removal efficiency of Cd2+ could be enhanced by increasing the initial pH, current density, and Na2SO4 dosage at initial DO concentration of 1.6 mg/L. However, a low current efficiency was also obtained when a high current density was applied because the high anodic potential resulted in water electrolysis. In brief, the generation of green rust (GR) promotes Cd2+ removal, particularly under anaerobic conditions, and Na2SO4 positively affects the flocculation and adsorption efficiencies of GR by strong charge neutralization. The obtained results can provide meaningful insights for the application of Fe-electrocoagulation in real industrial wastewater and understanding the mechanism of Fe based hydroxide coagulations for toxic metal removal.