Abstract: A device and method for treating urban domestic sewage based on a two-stage combined process of partial denitrification-anammox belong to the field of biological sewage treatment. The device includes a raw water tank, a sequencing batch biofilm reactor (SBBR), an intermediate water tank, an up-flow anaerobic sludge bed (UASB) and a water outlet tank. A part of urban domestic sewage enters the SBBR and is mixed with residual sewage in the last cycle, a partial denitrification-anammox reaction is carried out under a stirring condition to remove nitrate nitrogen and a part of ammonia nitrogen, followed by a nitrification under an aeration condition to completely convert ammonia nitrogen into nitrate nitrogen, and effluent enters the intermediate water tank; and the other part of the urban domestic sewage is mixed with the effluent of the SBBR and continuously enters the UASB, and nitrite nitrogen, which is generated by nitrate nitrogen reduction, and ammonia nitrogen, are removed by means of anammox.