The invention relates to a method for preparing functionalized carbon-coated magnetic nanoparticles. The method comprises the following steps: (1) dissolving ferrocene in acetone, and performing ultrasonic stirring so as to form a uniform solution; (2) adding polyacrylic acid into the uniform solution obtained in the step (1), and performing magnetic stirring so as to form a precursor; (3) performing a heated heat-preservation reaction on the precursor obtained in the step (2), performing cooling after the reaction is completed, removing the supernate, performing washing with acetone, and performing magnetic separation, thereby obtaining the functionalized carbon-coated magnetic nanoparticles. Compared with the prior art, the functionalized carbon-coated magnetic Fe3O4 nanoparticles prepared by using the method are synthesized by using a solvent heating method, the prepared magnetic submicrospheres are uniform in particle size, good in stability, free of aggregation among particles, and excellent in magnetic property.