The invention discloses a sewage treatment device, system and method. The sewage treatment device comprises a reaction column, an aerator, a water feeder and a water drainage device, wherein the reaction column is vertically arranged, the water feeder is arranged below the reaction column, the aerator is arranged at the bottom of the reaction column, the water drainage device is arranged above the reaction column, an activated sludge water purification region, a biofilm water purification region and a clear water region are arranged in the reaction column from bottom to top according to a water flow direction sequentially; and a bearing layer with the capabilities of water permeability and sludge isolation is arranged between the activated sludge water purification region and the biofilm water purification region. The sewage treatment system comprises a plurality of sewage treatment devices which are connected in series with one another. The sewage treatment method comprises the following steps: (1) a starting phase, namely, injecting activated sludge into the activated sludge water purification regions of the reaction columns in each sewage treatment device in the sewage treatment system, and carrying out smothering aeration; and (2) a continuous running phase, namely, regulating according to the following principles: the longer the running time is, the larger the water inlet quantity is, and the larger the aeration quantity is. The sewage treatment device, system and method are strong in loading ability, long in backwashing period and low in energy consumption and sludge discharge.