Environmental friendly magnetic Fe3O4/beta-FeOOH nanocomposites with low cost were prepared via a simple one pot method and their physiochemical properties were investigated. The Fe3O4/beta-FeOOH nanocomposites efficiently catalyzed the activation of peroxymonosulfate (PMS) for sulfamethoxazole (SMX) degradation and can be easily recovered through magnetic separation. The effects of catalyst dosage, PMS dosage, temperature and pH were evaluated. The catalyst showed great stability and reusability based on the successive degradation cycles. The reactive oxygen species (ROS) including sulfate radical (SO4-center dot), hydroxyl radical (center dot OH) and singlet oxygen (O-1(2)) were generated in the Fe3O4/beta-FeOOH/PMS system, while both of SO4-center dot and O-1(2) were dominantly attributed to the SMX degradation. The special tunnel-type structure and surface oxygen vacancies of beta-FeOOH may be responsible for the high catalytic activity towards PMS to degrade SMX. At last, the catalytic mechanism of PMS on the surface of catalysts were proposed.