In order to improve the adsorption capacity of chitosan and study the adsorption of diclofenac from aqueous solution by chitosan, magnetic chitosan beads were prepared by in-situ coprecipitation method and modified by ethylenediamine. The adsorption capacity of the modified magnetic chitosan (EMMCS-G) for removal of diclofenac in water was investigated with different dosages of cross-linking agents such as glutaraldehyde, epichlorohydrin, and ethylenediamine, and results show that the optimal adsorbent was EMMCS-G when the dosages of glutaraldehyde, epichlorohydrin, and ethylenediamine were 4,2 and 6 mL, respectively. EMMCS-G was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), Fourier transform infrared (FTIR), and X-ray diffraction (XRD). Effects of time, temperature, pH, adsorbent dosage, and regeneration cycles on the adsorption of diclofenac by EMMCS-G were investigated. Results show that Fe₃O₄ particles were successfully coated by chitosan. The number of amino groups of the adsorbent increased significantly after modification, and the modified adsorbent had strong magnetic capacity and reusability. The optimum pH was 6.5. When the initial concentration of diclofenac was 50 mg/L and the dosage of the adsorbent was 100 mg, the removal rate was above 76% after 12 hours of reaction. The adsorption process accorded with the pseudo second-order kinetic model, and the adsorption data fitted well with the Freundlich model. Based on the thermodynamic analysis and pH experimental results, it demonstrates that the adsorption process was endothermic and spontaneous, which was mostly controlled by physical and chemical interactions.