The development of high performance magnetic nanomaterials has a good application prospect in wastewater treatment. Herein, Fe3O4/C nanocomposites was synthesized by eco‐friendly hydrolysis‐calcination method using potassium fulvic acid (FA−K) and FeCl3⋅6H2O as raw materials. The morphology of Fe3O4/C nanocomposites exhibited a loose pore structure with uniform size (a diameter ∼ 27 nm). The prepared Fe3O4/C nanocomposites possessed excellent dispersibility and magnetic properties in aqueous solution. The removal efficiency and maximum adsorption capacity of Fe3O4/C nanocomposites were 98% and 64.0 mg g−1, respectively. Moreover, it was confirmed that the adsorption mechanism of Cr(VI) contains electrostatic attraction and redox reaction. In this work, we provide a convenient means for manufacturing Fe3O4/C nanocomposites, and prove that FA−K is a cheap carbon source for manufacturing Fe3O4/C nanocomposites, which makes Fe3O4/C nanocomposites have potential application value in the effective adsorption of Cr(VI).