A device for sewage treatment including a reactor and a magnetic field generator. The reactor includes a water inlet at the upper end, a water outlet on the side wall, and a sludge outlet at the lower end. A stirrer and an aeration device are disposed in the reactor. The reactor is filled with magnetic powder and activated sludge. The magnetic field generator includes a magnetic field tester, an iron cover, a power supply, a transformer, and a spiral coil. The spiral coil loops around the outer surface of the reactor. The transformer, the spiral coil, and the power supply are connected in sequence. The iron cover surrounds the periphery and the bottom of the reactor, and the magnetic field tester is disposed within the reactor. A method for sewage treatment using the device is also provided.