An apparatus for treating municipal sewage by using a partial nitrosation-anaerobic ammonia oxidation process is composed of an anaerobic ammonia-oxidizing bacterium enriching and culturing device anda continuous flow PN / A autotrophic biological denitrification device. The anaerobic ammonia-oxidizing bacterium enriching and culturing device comprises a water distribution tank, a liquid medicine barrel, an enriching and culturing reactor and a control system, the continuous flow PN / A autotrophic biological denitrification device comprises a water inlet pump, a PN / A reactor, a sedimentation tank, a sludge reflux pump and a sludge discharge pump, the water inlet pump pumps municipal sewage subjected to carbon removal pretreatment, an aeration device is arranged at the bottom of the PN / A reactor, the air pipe of the aeration device is connected with an air blower, the inlet of the PN / A reactor is connected with the water inlet pump, the outlet of the PN / A reactor is connected with the inlet of the sedimentation tank, a sludge discharge pipe at the bottom of the sedimentation tank is respectively connected with the sludge discharge pump and the sludge reflux pump, and a pipeline for connecting the sludge discharge pipe with the sludge reflux pump is connected to a pipeline for connecting the water inlet pump with the PN / A reactor. The apparatus has the advantages of high starting speed, low effluent nitrate concentration, stability in operation, and suitableness for biological denitrification of municipal sewage.