Magnetic nanomaterials are promising adsorbents for their superior performance in the elimination of radioactive ions from aqueous solution. In this study, a new kind of fluorescent carbon dots were successfully introduced into MnFe2O4 to fabricate magnetic polyethyleneimine-functionalized carbon quantum dots/MnFe2O4 (PECQDs/MnFe2O4) nanocomposites by a simple hydrothermal method, which combined the fluorescent characteristic of CQDs, adsorption capacity of polyethyleneimine and magnetic separation property of MnFe2O4. Detailed characterization certified the formation of nanohybrid composites and its integrating advantages of fluorescence property and magnetism separation, which demonstrated its potential in immobilizing U(VI) and monitoring the adsorption process simultaneously in wastewater. The obtained PECQDs/MnFe2O4 was applied to remove U(VI) from aqueous solution and showed superior construction stability and adsorption capacity of 194 mg/g. Moreover, the kinetic and thermodynamic results suggested that the immobilization of U(VI) was an endothermic and spontaneous multilayer adsorption dominated process. In the end, the cation exchange and interaction between uranyl ions and abundant functional groups (i. e., -NH2, -OH and -COOH) on PECQDs/MnFe2O4 were revealed to be responsible for the enhanced adsorption of U(VI), which was realized by forming outer-sphere surface complexes in the removal process. This study proposed an ultrafast-kinetics and high-efficiency adsorbent for U(VI) sequestration, which has a great application potential due to its outstanding recycling performance.