Abstract: The extraction of U(Ⅵ) from 1.0 mol/L NaNO₃ using di(tert-butylbenezene)dithiophosphinic acid(donated as HA) in xylene as the organic phase has been investigated, and the extracted complex has been structurally and thermodynamically analyzed. By revealing the effect of pH values, the concentration of \rmNO_\text3^- in aqueous phase, and the concentration of HA in the organic phase on the distribution ratio of U(Ⅵ) between the two phases, it is confirmed that the extraction is governed by the cation-exchange mechanism described by the following equation: \rmUO_2,\;\mathrma^2+ +2HA_o=UO₂A_2,o+2\mathrmH^+_\mathrma , where a and o in subscript represent the aqueous and organic phases. The extraction equilibrium constant(lg K_ex) is determined to be −2.29±0.09 at 25 ℃. Compared with the extraction of An(Ⅲ) and Ln(Ⅲ) by HA and dialkyl dithiophosphonic acid, it is found that the substituents have little influence on the extraction of U(Ⅵ), but have obvious influence on the extraction of An(Ⅲ) and Ln(Ⅲ). The relative content of water to the extracted U(Ⅵ) in the organic phase is determined by Karl-Ficher method, and the formation of single extracted complex species is confirmed by the unique Raman peak of O=U=O unit for the organic phase of varying U(Ⅵ) loading, then the molecular formula of the extracted complex is proposed as UO₂A₂•H₂O. Crystal of UO₂A₂•H₂O was prepared from the organic phase loaded with appropriated U(Ⅵ) with slow vaporization of the xylene solvent. X-ray single crystal diffraction analysis results show that the complex belongs to the triclinic system with P1 point group, and the unit cell parameters are: a=11.235 Å(1 Å =0.1 nm), b=15.806 Å, c=16.522 Å; α =61.612°, β=71.670°, γ=79.875°; Z=2. In the solid compound, each complex UO₂A₂•H₂O molecule exists as separated unit in which two A⁻ anion provide their sulfur atoms together with one water molecule coordinating with \textUO_\text2^\text2+ in the equatorial plan to form a common bi-pentapyramidal geometry in uranyl complexes. The U—S bond lengths of the extracted complex are 2.853, 2.842, 2.871 Å and 2.862 Å, and the U=O bond lengths of uranyl are 1.765 Å and 1.764 Å, which are close to those of other reported complexes of thiophosphonic acid ligands with uranyl, but the U—S bond of crystal of UO₂A₂•H₂O is more asymmetric.