The highly efficient enrichment of amidoxime/magnetic graphene oxide (AO/mGO) for U(VI) was elucidated by batch, EXAFS and modeling techniques. The characteristic results indicated that AO groups were successfully grafted to surface of mGO by plasma-grafting method. The batch experiments showed that AO/mGO composites presented the high enrichment performance (approximate 435, 55, 69, 43, 108 and 32 mg/g for U(VI), Th(IV), Eu(III), Sr(II), Pb(II) and Cs(I), respectively) and rapid adsorption rate (adsorption equilibrium within 30 min). The maximum enrichment capacity of AO/mGO composites for U(VI) from seawater of south China sea was 2.85 mg/g, indicating the potential for capturing U(VI) from seawater. The EXAFS analysis indicated that highly effective enrichment of AO/mGO composites for U(VI) was attributed to inner-sphere surface complexation due to the occurrence of U-Fe shell even at low pH. The results of surface complexation modeling indicated that U (VI) adsorption from simulated seawater on AO/mGO can be satisfactorily simulated by two inner-sphere surface complexes such as SOUO2+ and SOUO2 (CO3)(2)(3-) species. These findings are crucial for the application of AO-based composites in the preconcentration of U(VI) from aqueous solution in environmental cleanup and the recovery of uranium from seawater.