The invention which belongs to the technical field of radioactive post-treatment concretely relates to a method for separating U from Pu in a Purex process. The method comprises the following steps: 1, removing nuclear reaction splinters except U and Pu; 2, coextracting codecontaminated U and Pu to a TBP (tributyl phosphate)/kerosene mixed liquor with a volume percentage of 30%; and 3, adding a reducing agent HSC (hydroxysemicarbazide) to the mixed liquor in step 2, and fully mixing to back-extract Pu (III) which is reduced from Pu (IV) in an organic phase to an aqueous solution of hydrogen nitrate, wherein the ratio of the molar concentration of the HSC in an aqueous phase solution to the molar concentration of Pu in an organic phase solution is 5-60, the volume ratio of the organic phase solution to the aqueous phase solution is 1:1-6:1, the temperature is 21-42.5DEG C, and the concentration of H<+> is 0.3-1mol/L. A technical scheme provided by the invention allows Pu (IV) in the organic phase to be rapidly reduced and back-extracted to the aqueous phase, the reaction time is two magnitudes shorter than the reaction time of the prior art, the technological process is simplified, and the separation efficiency is improved.