Series of magnetic biochar (Fe-BC_0.72, Fe-BC_0.55 and Fe-BC_0.25) were prepared by solid-phase grinding coupled with pyrolysis of ferrous sulfate and red tea residue. The effects of mass ratio of Fe₃O₄ and BC, Fe-BC and persulfate (PS) dosages, solution pH and temperature on rhodanine B(Rh B) degradation were investigated in the system of Fe-BC and PS. The results showed that the degradation rate of Rh B was ideal as the mass ratio of Fe₃O₄ and BC was 0.55, Fe-BC_0.55 dosage was 0.021 3 g and PS mass was 0.030 2 g at pH5 and the reaction temperature of 30 ℃. The degradation rate of RhB was 98.8% at the reaction time of 90 min. The degradation rate of Rh B was 75% after BC-Fe_0.55 was recycled for 5 times via magnetic separation.