Abstract: In order to optimize the Purex process, the reduction stripping behavior of Pu(Ⅳ) from 30% TBP/OK with formoxmine (FO) was investigated. The effect of temperature, FO concentration, HNO₃ concentration, NO₃^- concentration, U concentration in organic phase, contacting-time and phase ratio was studied, respectively. The results show that the stripping percentage of Pu(Ⅳ) increases with the phase contacting-time, FO concentration, U concentration in organic phase and the temperature, and decreases with HNO₃ concentration, NO₃^- concentration. The separation of Pu from U by FO in 1B contactor was carried out using 16-stage counter current extraction (in which 4 stages for supplemental extraction, 12 stages for stripping). At phase ratio (1BF∶1BX∶1BS)=4∶1∶1, the recovery percentages of both U and Pu are more than 99.99%. The separation factor of Pu over U is 1.0×10⁴, and the separation factor of U over Pu  is 8.3×10⁴. The present work indicates that FO as a new organic reducing and complexing agent can effectively achieve the separation of Pu from U.