Abstract: Aromatic polyamide composite reverse osmosis membrane was selected to treat radioactive waste water. Sodium ions and calcium ions are key metal ions contained in the low and intermediate level radioactive waste water. The effect of sodium ions, calcium ions on the retention efficiency of non-active nuclides cobalt ions and membrane permeation flux were studied under different pH values and operating pressures. The study shows that Na⁺ and Ca²⁺ in the radioactive waste water can reduce the retention efficiency of cobalt, and the depression effect of Ca²⁺ on the removal of cobalt is greater than the effect of Na⁺. The optimum process parameters are determined as follows: pH=10, operating pressure greater than 1 MPa. The retention efficiency of cobalt in radioactive waste water is maintained up to 98%. The results show that reverse osmosis can effectively remove cobalt from radioactive waste water, and meanwhile get a stable membrane flux.