In this paper, magnetic core–shell nanoflowers Fe₃O₄@MnO₂ were fabricated via a solvothermal method. We demonstrated that the as-synthesized magnetic nanoflowers Fe₃O₄@MnO₂ possess intrinsic oxidase-like activity in a wide pH range and can catalytically oxidize 4-aminoantipyrine (4-AAP) and phenol substrates to form pink color products without the requirement for additional oxidizing agents. On the basis of this phenomenon, a simple colorimetric method for the determination of phenol was developed. A wide linear detection range can be obtained from 1.0 μM to 120 μM (R² = 0.9962) with a detection limit of 0.15 μM. And the method was applied to determine phenol in wastewater with good recoveries ranging from 96.0 to 101.5%. Furthermore, benefitting from chemical stability and easy recovery (by simple magnetic separation) of Fe₃O₄@MnO₂, the oxidase mimetics have excellent reusability and reproducibility in cycle analysis.