This work described a novel method for the synthesis of high-ferromagnetism nanoparticles (Fe3O4/CNTs) to efficiently remove Cr(VI) from aqueous solution. The Fe3O4/carbon nanotubes (CNTs) were prepared by in situ reduction with post-oxidation method by using cheap and environmentally friendly precursor under mild condition. Magnetic hysteresis loops revealed that Fe3O4/CNTs had superior saturation magnetization (152 emu/g), enabling the highly efficient recovery of Fe3O4/CNTs from aqueous solution by magnetic separation at low magnetic field gradients. FTIR, Raman, XPS, and TEM observations were employed to characterize the physical-chemical properties of Fe3O4/CNTs, demonstrating that CNTs were successfully coated with iron oxide matrix. The adsorption equilibrium of Cr(VI) on Fe3O4/CNTs was reached within 30 min. Langmuir, Freundlich, and Dubinin-Radushkevich isotherm were chosen to analyze the equilibrium data. The results indicated that Langmuir model can well describe the equilibrium data with the maximum adsorption capacity of 47.98 mg/g at room temperature and 83.54 mg/g at 353 K. The adsorption capacity of Fe3O4/CNTs for Cr(VI) was greatly improved as compared to raw CNTs and other similar adsorbents reported. The pseudo-second-order kinetic model provided the best description of Cr(VI) adsorption on Fe3O4/CNTs. Most importantly, possible synthesis mechanism and Cr(VI) removal mechanism were explored. The results suggest that large amounts of Cr(VI) were adsorbed on Fe3O4/CNTs surface by substituting the surface position of -OH and then reducing it to Cr(OH)3 and Cr2O3.