Abstract: The efficient separation between radionuclides and boric acid in boron-containing radioactive liquid waste of nuclear power plants is crucial for achieving radioactive waste minimization. In this study, a three-stage reverse osmosis (RO) separation system using cellulose acetate membrane modules was developed to separate radionuclides (with Cs⁺ as the representative nuclide) from boric acid in simulated radioactive liquid waste. The factors including feed solution pH, boric acid concentration, Cs⁺ concentration, and operation pressure on the permeation of Cs⁺ and boric acid were experimentally evaluated. The separation factors between Cs⁺ and boric acid were quantitatively determined under various conditions. Experimental results revealed that the optimized three-stage RO system achieved a Cs⁺ rejection rate exceeding 85% while maintaining a total boric acid permeation rate over 90%, and the maximum separation factor reaching 6.3. This performance demonstrates the three-stage RO unit could effective separation Cs⁺ contaminants from boric acid in the waste streams. Hot tests with actual radioactive solutions confirmed the practical efficacy of the three-stage RO unit, with significant removal of γ-emitting radionuclides alongside high boric acid recovery rate. The developed three-stage cellulose acetate RO process presents a technically viable treatment approach for nuclear power plant boron-containing liquid waste, simultaneously addressing the effective separation of radionuclides from boric acid.