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The yolk-shell Co3O4@Fe3O4/C nanocomposites with Co3O4 as the core, Fe3O4/C as the shell, and a cavity structure were synthesized by the hard template method. The physical and chemical properties of the composites were characterized by SEM, TEM, XRD, TGA, XPS, BET, and VSM. The specific surface area of yolk-shell Co3O4@Fe3O4/C nanocomposites is 175.9 m2 g-1, showing superparamagnetic properties. The yolk-shell Co3O4@Fe3O4/C nanocomposites were used as heterogeneous Fenton catalysts to activate peroxymonosulfate (PMS) to degrade MB, which showed high catalytic degradation performance. The degradation rate of MB reached 100% within 30 min under the circumstances of the yolk-shell Co3O4@Fe3O4/C nanocomposites dosage of 0.1 g L-1, the PMS dosage of 1.0 g L-1, the initial MB concentration of 100 mg L-1, an initial pH of 5.5, and a temperature of 30±2 ℃. The enhanced catalytic performance of the yolk-shell Co3O4@Fe3O4/C nanocomposites can be attributed to the synergistic effect of the two catalytically active materials and the middle cavity. The effects of different operating parameters and co-existing anion species on MB degradation were also investigated. Electron paramagnetic resonance (EPR) analysis and quenching experiments confirmed that the formation of SO₄̇⁻ in the yolk-shell Co3O4@Fe3O4/C/PMS system contributes to MB degradation. In addition, yolk-shell Co3O4@Fe3O4/C nanocomposites can be easily separated from the pollutant solution under the action of an external magnetic field, and the degradation rate of MB can still reach 98% after five cycles, indicating that it has good stability and reusability and has broad application prospects in the field of water purification.