Emissions of nitrous oxide (N₂O) occur during the biological denitrification process, but a few studies have focused on N₂O emissions during denitrification under endogenous conditions. To investigate the characteristics of N₂O emissions and accumulation during the process of endogenous denitrification, experiments were carried out to explore the action mechanisms of nitrite (NO₂^-) on N₂O production during the denitrification process, with intracellular polymers serving as the electron donor. The results indicated that the N₂O production factors increased with increasing initial NO₂^--N concentration due to the slow biological degradation of the intracellular polymers and the inhibitions of free nitrite acid (FNA) on the activity of nitrous oxide reductase (Nos). When the NO₂^--N concentration was further increased, exceeding 100 mg N/L, the N₂O production factors decreased, potentially on account of the inhibition of increased FNA on the activities of nitrite reductase (Nir) and nitric oxide reductase (Nor), resulting in declined nitrite removal efficiency and increased NO production. Studying the mechanisms of N₂O accumulation during endogenous denitrification is beneficial for controlling N₂O emissions during the biological nitrogen removal process in wastewater treatment.