Abstract: The separation of neptunium (Np) from spent nuclear fuel was accomplished by availably changing the oxidation state of Np from +Ⅵ to +Ⅴ during the plutonium uranium reduction extraction (Purex) process. Therefore, a lot of salt-free reductants were explored experimentally, including hydroxylamine, hydroxamic acid, aldehydes, hydrazine, oxime, and their derivatives. Acetaldoxime (CH₃CHNOH) effectively achieves the reduction from Np(Ⅵ) to Np(Ⅴ), but the reduction mechanism is not clear. Cis (Z) and trans (E) isomers of CH₃CHNOH exist in solution, which may have different reduction abilities and reaction processes for Np(Ⅵ). In this study, scalar relativistic density functional theory was used to investigate the reduction mechanism of Np(Ⅵ) by Z/E-CH₃CHNOH. One Z/E-CH₃CHNOH molecule can reduce two Np(Ⅵ) ions. The H atom of the hydroxyl group on the Z-CH₃CHNOH contacts with the neptunyl oxygen atom, which results in the first Np(Ⅵ) reduction and forms the Np(Ⅴ) and free radical Z-CH₃CHNO^•. Subsequently, the C atom of free radical Z-CH₃CHNO^• touches the O atom of the coordinated water molecule of the second Np(Ⅵ) species, with the formation of Np(Ⅴ) and 1, 1-nitrosoethanol (CH₃CH(OH)NO). Finally, CH₃CH(OH)NO dissociates into nitroxyl (HNO) and acetaldehyde (CH₃CHO) by intermolecular hydrogen transfer. E-CH₃CHNOH also achieves the reduction of two Np(Ⅵ) ions by similar processes of Z-CH₃CHNOH, but the complexation structures and energy barriers of Np(Ⅵ) with two isomers are different. The reduction process of Np(Ⅵ) by Z-CH₃CHNOH is thermodynamically more favorable than that of E-CH₃CHNOH based on their potential energy profiles (PEPs), probably due to the formation of more hydrogen bonds in the former. The reduction of the first and second Np(Ⅵ) by Z-CH₃CHNOH needs to overcome the 22.36 and 19.19 kcal/mol energy barrier, respectively, which suggests that the first Np(Ⅵ) reduction is the rate-determining step. E-CH₃CHNOH overcomes the energy barriers of 21.17 and 23.03 kcal/mol for the first and second Np(Ⅵ) reductions respectively, indicating that the second Np(Ⅵ) reduction is the rate-determining step. These results clarify that Z-CH₃CHNOH and E-CH₃CHNOH have nearly identical reduction abilities to Np(Ⅵ). The results of bond distance and localized molecular orbitals indicate that the reduction process of both Np(Ⅵ) by Z/E-CH₃CHNOH is accompanied by the breakage and formation of related bonds. The reduction nature of the first and second Np(Ⅵ) reduction belongs to hydrogen atom transfer and water-participated single electron transfer respectively, which is also confirmed by the values of spin density and Np-O_yl bond distance. This work elucidates the reduction nature of Np(Ⅵ) by Z/E-CH₃CHNOH, which provides a theoretical basis for the Np separation in spent nuclear fuel.