A method for sewage treatment using a device, the device comprising: a) a reactor, the reactor comprising a stirrer, an aerator, an activated sludge zone, an upper end comprising a water inlet, a side wall comprising a water outlet, a bottom comprising a sludge outlet; and b) a magnetic field generator, the magnetic field comprising magnets and iron plates; wherein: the activated sludge zone is disposed inside the reactor; the stirrer and the aerator are disposed within the activated sludge zone, and 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 an inner side of the corresponding iron plate; and the magnets disposed on both sides of the reactor produce magnetic fields having the same direction; the method comprising the following steps: a) filling an activated sludge inside the reactor to form the activated sludge zone in the bottom of the reactor; mounting the magnets in parallel on two side of the reactor for allowing the magnets to form magnetic fields having the same direction in the reactor; controlling an area of each magnet to ensure the activated sludge zone to be within ranges of the magnetic fields; and mounting iron plates outside corresponding magnets; b) inoculating the activated sludge at a temperature of between 4 and 15°C, introducing a simulated domestic sewage into the reactor, increasing an organic load from 0.03 kg /(m 3 ·d) to 0.8 kg /(m 3 ·d) at a constant gradient; jet aerating and stirring the activated sludge zone to domesticate the activated sludge in the bottom of the reactor; c) allowing the reactor to work in the form of sequencing batch, domesticating the activated sludge for 10-15 d, reversing the direction of each of the magnets on two sides of the reactor to strengthen the activated sludge zone within the magnetic fields of the reversed direction, domesticating the activated sludge for another 10-15 d, and reversing the direction of the magnets again; and repeating the process for between 4 and 5 times until a COD removal rate is 80% and maintains a stable state; and d) after between 1 and 2 months of sludge domestication, introducing a domestic sewage after being filtered by a grid into the reactor, controlling the organic load at 0.4-1.0 kg /(m 3 ·d); reversing the direction of the magnets for every 10-15 d during the sewage treatment to form an intensified sewage treatment system comprising the magnetic field and microorganisms. a) a reactor, the reactor comprising a stirrer, an aerator, an activated sludge zone, an upper end comprising a water inlet, a side wall comprising a water outlet, a bottom comprising a sludge outlet; and b) a magnetic field generator, the magnetic field comprising 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, and 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 an inner side of the corresponding iron plate; and the magnets disposed on both sides of the reactor produce magnetic fields having the same direction; a) filling an activated sludge inside the reactor to form the activated sludge zone in the bottom of the reactor; mounting the magnets in parallel on two side of the reactor for allowing the magnets to form magnetic fields having the same direction in the reactor; controlling an area of each magnet to ensure the activated sludge zone to be within ranges of the magnetic fields; and mounting iron plates outside corresponding magnets; b) inoculating the activated sludge at a temperature of between 4 and 15°C, introducing a simulated domestic sewage into the reactor, increasing an organic load from 0.03 kg /(m 3 ·d) to 0.8 kg /(m 3 ·d) at a constant gradient; jet aerating and stirring the activated sludge zone to domesticate the activated sludge in the bottom of the reactor; c) allowing the reactor to work in the form of sequencing batch, domesticating the activated sludge for 10-15 d, reversing the direction of each of the magnets on two sides of the reactor to strengthen the activated sludge zone within the magnetic fields of the reversed direction, domesticating the activated sludge for another 10-15 d, and reversing the direction of the magnets again; and repeating the process for between 4 and 5 times until a COD removal rate is 80% and maintains a stable state; and d) after between 1 and 2 months of sludge domestication, introducing a domestic sewage after being filtered by a grid into the reactor, controlling the organic load at 0.4-1.0 kg /(m 3 ·d); reversing the direction of the magnets for every 10-15 d during the sewage treatment to form an intensified sewage treatment system comprising the magnetic field and microorganisms.