The oxidation state greatly affects the chemical behavior of uranium in the nuclear fuel cycle and in the environment. Phosphonate ligands, on the other hand, show strong complexation toward uranium at different oxidation states and are widely used in nuclear fuel reprocessing. Therefore, in this work, the reduction behavior of U(VI) with the presence of a phosphonate ligand is investigated under mild solvothermal conditions. By adjusting the reaction time, temperature, and counterion species, a series of uranium diphosphonates including two U(VI), seven U(IV), two mixedv-alent U(IV/VI), and one distinct U(IV/V/VI) compounds were obtained. All these compounds were characterized by single crystal X-ray diffraction and UV-vis-NIR absorption and fluorescence spectroscopy. The structural diversity among those compounds not only illustrates the intrinsic structural complexity in this system, but also illuminates the in situ reduction pathways that are affected by the variation of reaction conditions. The UV vis NIR absorption spectra of the tetravalent uranium compounds show that the absorption features are closely related to the local coordination environments of the uranium centers as well as the bonding modes of the phosphonate ligand. The fluorescence spectra of mixed-valent uranium compounds show unique emission features of U(VI) luminescence that are partially quenched by the multiple electronic transitions of U(IV) centers in the visible and NIR regions.