The invention discloses a low-concentration sewage treatment system which comprises an aerating and activating tank connected with a water inlet pipeline. The aerating and activating tank is sequentially connected with a contact backflow tank, a sedimentation tank and a water outlet pipeline in the water flow direction, a filler device is arranged in the contact backflow tank, an aerating system is arranged in the aerating and activating tank, and a flow guide structure which can enable a water body to circulate and flow between the aerating and activating tank and the contact backflow tank under aerating action is arranged between the aerating and activating tank and the contact backflow tank. The low-concentration sewage treatment system disclosed by the invention fully utilizes advantages of two technologies of an activated sludge method and a contact oxidation method, not only is beneficial to culture and growth of microorganisms, but also improves an oxygenating and aerating effect, avoids sludge backflow, reduces operation energy consumption and management difficulty and is especially suitable for low-concentration sewage treatment.