A novel porous magnetic adsorbent with sufficient interconnected pore structure of chitosan- g-poly(2-acrylamide-2-methylpropane sulfonic acid) (CTS-g-AMPS) was prepared by grafting AMPS onto CTS via Pickering high internal phase emulsions (Pickering-HIPEs) polymerization, which stabilized with the modified Fe3O4 nanoparticles, and used for the removal of the cationic dyes methylene blue (MB), brilliant green (BG), and methyl green (MG) from aqueous solution. The effects of initial pH, contact time, and initial concentration on the adsorption properties of the porous adsorbent were studied systematically. It was revealed that the porous magnetic adsorbent can rapidly adsorb MB, BG, and MG with high adsorption capacities within a wide pH range from 6.0 to 12.0. The maximum adsorption capacities can be reached to 1,044.56, 1,625.94, and 908.01 mg/g for MB, BG, and MG, respectively, within 20 min. After adsorption of MB, BG, and MG, the porous magnetic adsorbent can be recycled easily from the solution by a magnet, and regenerated for reuse. The adsorption capacities of the adsorbent still reached 1,010.02, 1,578.73, and 889.01 mg/g for MB, BG, and MG, respectively, after five adsorption-desorption cycles. The recyclable magnetic adsorbent showed high adsorption capacity, fast adsorption rate, and excellent reusability, and can be used as a potential adsorbent for the decontamination of dye-polluted water.