Abstract: In this paper, the feasibility of plutonium separation from the solution of irradiated neptunium target by solvent extraction with TBP was studied. The possibility of Pu(Ⅳ)-Np(Ⅳ) combination as extracted valence combination was investigated from material liquid preparation and process design. The two-step method of UDMH reduction-sodium nitrite oxidation to control the valence state of Pu(Ⅳ)-Np(Ⅳ) was studied. The results show that more than 99.9% Pu(Ⅳ) and 99.5% Np(Ⅳ) can be kept stable for more than 4 h. Based on this, an extraction process for plutonium separation from the solution of irradiated neptunium target is designed and verified by cascade experiments. According to the designed process, the yield of neptunium in 1A is 99.5%, the stripping rate of neptunium in 1B is 0.8%, and the stripping rate of plutonium is 99.9%. The stripping rate of neptunium in 1C is 99.5%. The data show that the separation of neptunium and plutonium can be realized by controlling the extracted valence combination of Pu(Ⅳ)-Np(Ⅳ) in feed, and yet the problems of long time stability of valence states and the weak extraction ability for neptunium with TBP will affect the practical application of the process.