A device for sewage treatment, including: a reactor and a magnetic field generator. The reactor includes a stirrer, an aerator, an activated sludge zone, an upper end including a water inlet, a side wall including a water outlet, and a bottom including a sludge outlet. The magnetic field includes magnets and iron plates. The activated sludge zone is disposed inside the reactor. The stirrer and the aerator are disposed within the activated sludge zone. The aerator is arranged beneath the stirrer. The magnetic field generator is disposed outside the reactor. The magnets and the iron plates are symmetrically disposed on two sides of the reactor, respectively, and each of the magnets is disposed on the inner side of the corresponding iron plate. The magnets disposed on both sides of the reactor produce magnetic fields having the same direction.