Eucalyptus-based magnetic activated carbon was prepared with KOH as activator and FeCl3•6H2O as magnetic agent by one-step method. The structure and properties of the sample were characterized by automatic analyzer of specific surface area and aperture analyzer, FTIR, XRD, and VSM. The adsorption properties of methyl orange on eucalyptus-based magnetic activated carbon were investigated. The effects of adsorbent dosage, initial concentration of methyl orange, pH value and adsorption time on the adsorption effect were investigated. The results showed that the BET specific surface area, pore volume, and average pore diameter of the prepared magnetic activated carbon MAC-0.42 were 1430.32 m2/g, 0.893 cm3/g, and 2.49 nm, respectively. The adsorption kinetics and adsorption isotherm experiments were conducted under the conditions of adsorbent dosage 0.045 g, initial mass concentration of methyl orange 0.25 g/L, pH of the solution (about 6.82), and adsorption time 10 h. The adsorption values of iodine and methylene blue of eucalyptus magnetic activated carbon were 1571.4 mg/g and 315.52 mg/g, respectively. There were —OH，—C=O, and —COO on the surface of the magnetic activated carbon.The sample contained magnetic substances Fe and Fe3O4 with a specific saturation magnetization of 48.65 emu/g, which could be separated from the solution quickly when the magnetic field was applied. The adsorption of methyl orange was consistent with the Langmuir model, and the maximum adsorption capacity was 333.33 mg/g. The adsorption process was a spontaneous endothermic process. The dynamic adsorption behavior of methyl orange was more consistent with the pseudo-second-order kinetic model.