In this study, nitrogen removal performance of the denitrifying ammonium oxidation (DAO) process was investigated when treating sulfamethoxazole (SMX)-laden secondary wastewater effluent. The influent SMX concentration showed negligible effect on efficiencies for removal of nitrate and COD. However, the ammonium ions removal rate was moderately reduced, when the influent SMX concentration in wastewater reached 6 mg/L. Total nitrogen removal efficiency remained as high as 76.77％ towards the day 158 at the end of experiment. Candidatus_Brocadia and Candidatus_Kuenenia were the functional anammox strains. The unclassified_f__Rhodobacteraceae sp. was predominant heterotrophic denitrifying strain in the studied reactor. The concentrations of soluble extracellular polymeric substances in sludge obviously increased from 16.76 mg/g VSS to 32.31 mg/g VSS, which might protect the nitrogen removal strains from high-concentration SMX. This result provides a theoretical and technical foundation for the application of denitrifying ammonium oxidation process in treating sulfamethoxazole-laden secondary wastewater effluent.