A draining caisson comprises a caisson body, a platform sewage draining pipeline, a sewage outlet and a plurality of inclined plates, wherein the platform sewage draining pipeline is arranged above the inside of the caisson body, the sewage outlet is arranged below the sewage draining pipeline, the inclined plates are arranged below the sewage outlet and on two sides of the inside of the caisson and distributed in a crisscross manner, the sewage outlet is arranged on a side wall of the sewage draining pipeline, and the bottom of the sewage draining pipeline is sealed. Velocity of sewage impacting with the inclined plates can be shortened, oil and water separation is facilitated, and can be separated from each other after staying inside the caisson for enough time, so that the best oil and water separation effect of the draining caisson can be achieved, oil content of sewage outfall media of the draining caisson can be reduced, sewage outfall is realized, while marine pollution caused by offshore petroleum platforms is reduced.