Fly ashes generated by the incineration disposal of radioactive pollutants contain a large amount of radionuclide, such as Cs-137. The radioactive cesium can be removed from these ashes by water washing. However, this method produces a large amount of radioactive contaminated wastewater. In this study, we investigated the effect of coexistent ions on Cs-137(+) rejection of polyamide RO membrane in the decontamination of wastewater with low level radioactivity. The Cs-137(+) rejection increased with the increase of monovalent cations concentration, while it decreased with the increase of bivalent cations concentration. However, the effect of bivalent cations was diminished by coexistent monovalent cations in the treatment of model wastewater that simulated the washing water of radioactive fly ashes. In addition, the polyamide RO membrane had a high resistance for long-term radiation of Cs-137. Thus, a polyamide RO membrane shows sufficient performance in the decontamination of wastewater with low level radioactivity.