A series of poly(arylene ether nitrile) containing versatile carboxyl and sulfonic groups (CSPEN) were magnetically functionalized by ferroferric oxide (Fe3O4), and the obtained magnetic adsorbents (Fe3O4/CSPEN) were used to remove methylene blue (MB) dye from aqueous solution. The systematical characterizations that including scanning electron microscope (SEM), X-ray powder diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), Fourier transform infrared (FTIR) certified that the Fe3O4/CSPEN possessed versatile functional groups and magnetic separation properties. The batch adsorption studies revealed that the Fe3O4/CSPEN not only displayed high selective adsorption ability for cationic MB in the presence of anionic MO, but also exhibited a removal efficiency as high as 98.2%. Besides, the adsorption kinetics and isotherm matched well with pseudo-second-order model and Langmuir model, respectively, and the maximum adsorption capacity of Fe3O4/CSPEN for MB was 92.029 mg/g. The FTIR and Brunauer-Emmett-Teller (BET) analyses confirmed that the outstanding adsorption capacity of Fe3O4/CSPEN was attributed to their own microporous structure and the electrostatic interaction with MB. Therefore, the modified magnetic adsorbent can be used to selectively remove cationic dye from aqueous solution.