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A chitosan-coated-magnetite with covalently grafted polystyrene (g-PS) based magnetic carbon nanocomposites composed of carbon coated magnetite nanoparticles with carbonized g-PS microsphere are prepared for effective hexavalent chromium (Cr(VI)) adsorption. The adsorption measurements have indicated that the fabricated magnetic nanocomposites FC515 exhibit an effective Cr(VI) adsorption performance and the optimal pH for Cr(VI) adsorption on FC515 is 3.0 at 298 K. The Cr(VI) adsorption kinetics on FC515 is found to follow the pseudo-second-order behavior with a room temperature initial adsorption rate of 7.106 g mg^-1 min^-1 for the solution with an initial Cr(VI) concentration of 6.5 mg L^-1and pH value of 3.0. The adsorption isotherm test illustrates that the Langmuir isotherm model with a monolayer adsorption is well fitted for Cr(VI) adsorption on FC515 than the Freundlich isotherm model. The thermodynamic parameters including negative standard Gibb''s free energy change and standard enthalpy change suggest that the Cr(VI) adsorption on FC515 is spontaneous and exothermic. After 5 cycles, FC515 still maintain 86% of Cr(VI) adsorption capacity, exhibiting a good reusability in practice. The zeta-potential and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the Cr(VI) adsorption on FC515 is mainly from the electrostatic interaction between FC515 and Cr(VI). This work provides a new method to design the novel structure of magnetic carbon nanocomposites with natural polymer chitosan and thermoplastic PS for Cr(VI) wastewater treatment.