In this work, magnetic porous nitrogen doped carbon structures containing well-dispersed active Fe nanocrystals (Fe/N-C) are fabricated conveniently via Fe3+-mediated polymerization of dopamine as precursor combined with in situ post carbonization process, thus allowing the entire encapsulation of active Fe nanocrystals in the interior. The obtained functional hybrid materials at 700 degrees C (Fe/N-C-700) show spherical structure, high proportion of metallic Fe nanocrystals, ultrahigh surface area, and easy magnetic separation property, affording excellent U(VI) removal capability (232.54 mg.g(-1)), surpassing the Fe/N-C samples pyrolyzed at different temperatures and nano zero-valent iron. During the adsorption process, effects of water chemistries (i.e., reaction time, pH, carbonates concentration and temperature) on U(VI) adsorption on Fe/N-C-700 are full explored, and the well-dispersed Fe nanocrystals play important role in reducing into U(IV). Meanwhile, nitrogen dopant could make function well in the electrostatic interaction and partial reduction of U(VI). The present study demonstrates that Fe/N-C-700 nanospheres derived from Fe-PDA have potential application for the preconcentration and immobilization of U(VI).