The invention discloses an underground oxidation ditch sewage treatment system relating to a new oxidation ditch sewage treatment system which can be used for treating sewage in factory areas suffered from organic pollution in new districts in cities and small towns. An underground oxidation ditch consists of an underground annular closed ditch buried in a block or a factory area; sewage enters a three-grid septic tank with a grizzly screen and a sand setting function through a building drain system and then is connected to the oxidation ditch through a drain branch pipe; and water inlet points are distributed along the whole length of the underground oxidation ditch, so that water can enter from a plurality of points. The pretreated sewage in the factory area can be connected to the underground oxidation ditch. The sewage flows in the ditch in a gravity form; conduit type aerators are arranged at a certain interval, so that double functions of aerating to oxygenate and lifting sewage can be achieved, further, the sewage is used for cultivating active sludge microorganism, and the circulating flow can be realized. Water drained from the oxidation ditch enters a drain tank and is lifted to a secondary sedimentation tank through a sewage pump; and reflow sludge from the secondary sedimentation tank is reflowed to the oxidation ditch through a gravity pipeline.