Polymers of intrinsic microporosity shows great potential for dye adsorption and magnetic Fe₃O₄ are easy to be separated. In this work, hydrolyzed polymers of intrinsic microporosity-1/Fe₃O₄ composite adsorbents were prepared by phase inversion and hydrolysis process for cationic dye adsorption. The chemical structure and morphology of the composite adsorbents were systematically characterized by several characterization methods. Using methylene blue as the target dye, the influences of solution pH, contact time, initial dye concentration, and system temperature on the methylene blue adsorption process were investigated. The incorporation of Fe₃O₄ particle into hydrolyzed polymers of intrinsic microporosity-1 endow the adsorbent with high magnetic saturation (20.7 emu·g^–1) which allows the rapid separation of the adsorbent. Furthermore, the adsorption process was simulated by adsorption kinetics, isotherms and thermodynamics to gain insight onto the intrinsic adsorption mechanism. In addition, the composite adsorbents are able to selectively adsorb cationic dyes from mixed dyes solution. Hydrolyzed polymers of intrinsic microporosity/Fe₃O₄ shows only a slight decrease for methylene blue adsorption after 10 adsorption/regeneration cycles, demonstrating the outstanding regeneration performance. The high adsorption capacity, outstanding regeneration ability, together with simple preparation method, endow the composite adsorbents great potential for selective removal of cationic dyes in wastewater system.