Siliceous mesocellular foams (MCFs) with high surface area, large pore volume and ordered mesoporous structure have a great potential in the capture of pollutants. To achieve the sorption of U(VI) with enhanced capacity and selectivity, amidoxime groups were modified on MCFs by optimizing the molar ratio of (2-cyanoethyl) triethoxysilane (CPTES) in silicon source from 0% to 30%. CPTES promoted the grafted amounts of amidoxime group (0–1.85 mmol/g) while resulted in the collapse of mesopores of amidoximated MCFs from 426.5 to 136.3 m2/g. Sorption behaviors of U(VI) at the interface of amidoximated MCFs and solutions were systematically investigated under various hydrochemical conditions. Amidoximated MCFs could capture U(VI) rapidly (4.0 h) with superior maximum sorption capacity (446.6 mg/g at pH 5.0 and T 298 K), and favorable reusability (67.8% of capacity remained after six recycles). Selective enrichment of U(VI) from artificial nuclide wastewater (201.5 mg/g of capacity, 16.08–33.61 of selectivity coefficients) and seawater (179.3 mg/g of capacity, 5.08–20.48 of selectivity coefficients) against the interference of heavy metals and nuclides were presented. This study provided a valuable reference for the preparation of sorbents with surface amidoximation and advanced porosity as well as a meaning option for the extraction of nuclides from aqueous solutions.