The invention discloses a reaction-sedimentation tank. The reaction-sedimentation tank comprises a reservoir, a water outlet pipe, a sludge storage bucket, a water distribution pump, a sedimentation tank, a water inlet pipe, a water pipe, a multitubular mixer, a chemical feeder, an inclined plate, a grid, inclined pipes and a sludge discharge pipe. Characteristics of two flow states including plug flow and complete mixing are combined, after sewage enters the sedimentation tank, a flocculating agent is rapidly and uniformly mixed with mixed liquor in a reactor under the action of the multitubular mixer, a closed circulation type reaction tank shows a plug flow feature in short time and shows a complete mixing feature in long time, so that the buffer capacity of the reactor is improved, grid reaction is adopted in the reaction tank and adapts to the flow velocity from fast to slow, and the reaction is improved remarkably while compared with that by means of a traditional reaction-sedimentation tank; with adoption of the inclined plate and the inclined pipes for sedimentation, the sedimentation effect is improved, and the bearing capacity is improved greatly; besides, with adoption of the compact design concept, the occupation area and the investment cost are reduced.