Novel magnetic MnO2/MnFe2O4 nanocomposite was successfully fabricated by hydrothermal method. Subsequently, this nanocomposite was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microcopy (TEM), X-ray photoelectron spectra (XPS), vibrating sample magnetometer (VSM) and Brunauer-Emmett-Teller (BET) measurements. Then the material was applied to activate PMS for Rhodamine B (Rh B) degradation. The results displayed that the decomposition efficiency of the MnO2/MnFe2O4 nanocomposite with a mole ratio of 7:1 was 90% for Rh B within 5 min in the presence of PMS, higher than other MnO2/MnFe2O4 ratio, homogeneous catalysts (Fe3+ and Mn2+), pure MnO2 and pure MnFe2O4. In addition, influences of pivotal factors (PMS concentration, catalyst dosage and initial pH value), performance of different processes and reusability of MnO2/MnFe2O4 nanocomposite were also evaluated. Radical scavenging experiments were conducted and SO4 center dot- was indicated as the primary radical. What's more, the MnO2/MnFe2O4 nanocomposite could be convenient to recycle in practical application benefited from its magnetism. Finally, according to the results and previous studies, the believable mechanism of activating PMS by MnO2/MnFe2O4 nanocomposite was put forward, which could occupy an important position as a fast treatment option in sewage disposal.