The natural crab shell with the unique structure consisting of CaCO3 was employed as the biotemplate encapsulating urea to form porous g-C3N4 for U(VI) elimination. A multiscale characterization of porous g-C3N4 provided evidences for successfully introducing porous structure with increased surface area as well as multisorption sites (e.g. C-N=C, N-(C)(3) and C-N-H groups). The interaction of U(VI) with porous g-C3N4 was investigated by batch experiments, spectroscopic analyses and theoretical calculations. The sorption experiments indicated that g-C3N4-550 with the highest polymerization exhibited rapidly sequestering U(VI) within 120 min and superior uptake performance (149.70 mg/g) at pH = 5.0. The presence of U-N shell (R = 2.80 angstrom) calculated from EXAFS spectra revealed the U(VI) immobilization on g-C3N4-550 at pH = 5.0 was ascribed to the inner-sphere surface complexation. The DFT calculations further evidenced the strong interaction between uranyl and g-C3N4 (E-ad = 156.83 kcal/mol), and the most effective sorption site was inside the holes of g-C3N4. The superior uranium sorption ability along with low-cost and environmental friendly raw materials pointed to the high potential of porous g-C3N4 for uranium preconcentration. The findings will expand an exciting direction towards the effective removal of actinides in environmental cleanup.