As one of the most important and basic magnetic materials, Fe₃O₄ nanoparticles have wide applications in water treatment. In this paper, the Fe₃O₄ nanoparticles were synthesized by solvothermal method. The magnetic Fe₃O₄ nanoparticles were obtained with sizes ranging from about 10 to 150 nm, with a distinct anti-spinel structure and an abundance of hydroxyl groups on the surface. Moreover, the saturated magnetization and the coercivity of the Fe₃O₄ nanoparticles are 84.08 emu g⁻¹ and 89.52 Oe, respectively. This confirms that it has a good magnetic property compared with the commercial Fe₃O₄ nanoparticles. And then, the magnetic Fe₃O₄ nanoparticles were cultured biochemically with activated sludge to form the magnetic flocculent clusters for water treatment. The results show that the settling performance is excellent, and the sludge activity is well maintained, with microorganisms still abundant. Besides, the recovery rate of the magnetic flocculants was 95%.