The work aims to synthesize Fe3O4 nanoparticles in one-step method, analyze the processes of the formation of their core shells, and study the application of such nano-materials in magnetic anti-counterfeiting ink. Firstly, Fe3O4@C nanoparticles with a core-shell structure were synthesized. With FeCl3•6H2O as iron source, ethylene glycol as solvent, glucose as carbon source and urea as the alkali source, X-ray diffraction, field emission scanning electron microscope, high resolution transmission electron microscope, infrared spectrometer and laser Raman spectroscopy, etc. were employed to characterize the surface profile and structure of Fe3O4@C nanoparticles. Then, viscosity, friction resistance, fineness and magnetism of the prepared magnetic ink were tested. The prepared Fe3O4@C nanoparticles were the monodisperse spherical nanoparticles with Fe3O4 (mean grain size: 18 nm) as the core and 2 nm thick amorphous carbon as the shell, in which glucose played a key role in the formation of the core-shell structure. In addition, Fe3O4@C showed a typical soft magnet at room temperature with the saturation magnetization of 71.2 A•m-2/kg and coercive force of 10 984.8 A/m. For the prepared magnetic ink with magnetic mark, abrasion resistance of 81% and viscosity of 95 Pa•s, all its performances meet the requirements of magnetic anti-counterfeiting ink.