Abstract: The 1B unit is a critical separation unit in the Purex process, primarily responsible for the efficient separation of uranium (U) and plutonium (Pu). The separation principle is to use a reducing agent to reduce Pu(Ⅳ) to Pu(Ⅲ) that is not easily extracted by 30%TBP-kerosene or n-dodecane, so that Pu(Ⅲ) remains in the aqueous phase, thereby achieving the separation between uranium and plutonium. To elevate the plutonium concentration ratio in the 1B unit, simulation calculations were performed on 4-fold, 5-fold, and 6-fold Pu concentration processes using dimethylhydroxylamine (DMHAN) and monomethylhydrazine (MMH) as composite reducing agents. Additionally, a 16-stage counter current stripping experiment for the 6-fold Pu concentration process was conducted at 25 ℃ to verify the simulation results. Simulation results reveal that under the 6-fold Pu concentration condition, when 99.99% of Pu(Ⅳ) in the first-stage organic phase is reduced, the total oxidation rate of Pu(Ⅲ) by nitric acid (HNO₃) and nitrous acid (HNO₂) in both aqueous and organic phases remains lower than the two-phase reaction rate of trace Pu(Ⅳ) in the organic phase with DMHAN in the aqueous phase, ensuring effective reduction of Pu. Meanwhile, the recovery rates of U and Pu reach 99.999% and 99.995% respectively, with Pu/U separation factor (SF_Pu/U) of 2.0×10⁴ and U/Pu separation factor (SF_U/Pu) of 2.3×10⁵. Countercurrent experimental data confirm that at 6-fold Pu concentration, the U recovery rate is 99.999%, Pu recovery rate is 99.988%, SF_Pu/U is 7.3×10³, and SF_U/Pu is 1.4×10⁵, showing good consistency with simulation results. These findings demonstrate that adopting DMHAN-MMH as the reducing agent in the 1B unit can safely raise the Pu concentration ratio from the current 4-fold to 6-fold, providing technical support for optimizing the Purex process.