Fe₃O₄@C nanoparticles were prepared by an in situ, solid-phase reaction, without any precursor, using FeSO₄, FeS₂, and PVP K30 as raw materials. The nanoparticles were utilized to decolorize high concentrations methylene blue (MB). The results indicated that the maximum adsorption capacity of the Fe3O4@C nanoparticles was 18.52 mg/g, and that the adsorption process was exothermic. Additionally, by employing H₂O₂ as the initiator of a Fenton-like reaction, the removal efficiency of 100 mg/L MB reached ~99% with Fe₃O₄@C nanoparticles, while that of MB was only ~34% using pure Fe₃O₄ nanoparticles. The mechanism of H₂O₂ activated on the Fe₃O₄@C nanoparticles and the possible degradation pathways of MB are discussed. The Fe₃O₄@C nanoparticles retained high catalytic activity after five usage cycles. This work describes a facile method for producing Fe₃O₄@C nanoparticles with excellent catalytic reactivity, and therefore, represents a promising approach for the industrial production of Fe₃O₄@C nanoparticles for the treatment of high concentrations of dyes in wastewater.