The degradation of reactive black 5, an azo dye, in aqueous solutions was investigated using novel magnetic nano-Fe₃O₄@α-MnO₂catalyst, which prepared by two step hydrothermal method. The catalyst was characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) and vibration sample magnetometer (VSM). TEM and XRD results of as-synthesized catalyst showed the nano-α-MnO₂ coated Fe₃O₄ was successfully prepared. VSM indicated the saturation magnetization of nano-Fe₃O₄@α-MnO₂ was reach up to 39.89emu/g. As all the Fe₃O₄, α-MnO₂ and nano-Fe₃O₄@α-MnO₂ was applied to assess the individual and interaction effects of metals, the higher catalytic efficiency of nano-Fe₃O₄@α-MnO₂ implied the synergistic effect between Fe and Mn. Several operating parameters (catalyst dosage, PMS concentration and initial pH value) on the treatment efficiency and reaction kinetics of RBK5were also studied. As results, the RBK5degradation process via Fe₃O₄@α-MnO₂ activated PMS is consistent with the pseudo-first-order reaction. The degradation efficiency of RBK5 (30mg/L) could reach 91% within 60min under the condition of the catalyst dosage was 1.2g/L, the PMS concentration was 4mmol/L, and the initial pH value was 7.0. Under this situation, the degradation rate constant of RBK5also reached the highest value of 0.023min^-1. Moreover, the main active species in RBK5degradation in Fe₃O₄@α-MnO₂/PMS system was identified as SO₄^-· by adding radical quencher such as methanol, tert-butanol and nitrobenzene.