An apparatus for treating municipal sewage by anaerobic/aerobic/anoxic (AOA) [1] process via simultaneous endogenous partial [2] denitrification coupled with anammox in anoxic zone is disclosed. The apparatus mainly includes a raw water tank (1) for sewage, an AOA reactor (2) and a sedimentation tank (3), the sludge flows back from the bottom of the sedimentation tank (3) to the anoxic zone (2.4) and the anaerobic zone (2.2) respectively, and biofilm filler is added to the anoxic zone (2.4). The sewage enters the AOA reactor (2), and the intracellular carbon source is stored in the anaerobic zone (2.2) to remove the organic matter in the raw water. Then it enters the aerobic zone (2.3) for nitrification, and the generated nitrate-nitrogen enters the anoxic zone (2.4) for endogenous partial denitrification. The filler in the anoxic zone (2.4) uses the generated nitrite-nitrogen by endogenous partial denitrification and the remaining ammonia-nitrogen in the raw water to perform anammox reaction. The generated nitrate-nitrogen can be further removed by endogenous denitrification in the anoxic zone (2.4). Endogenous partial denitrification coupled with anammox is used for nitrogen removal in the anoxic zone (2.4), which can reduce the requirement of aeration in the aerobic zone (2.3) and the carbon sources in the anoxic zone (2.4), and suitable for low C/N ratio municipal sewage treatment. A method for treating municipal sewage by AOA process via endogenous partial denitrification coupled with anammox in anoxic zone is also provided.