Hydrazine is an important intermediate for anaerobic ammonium oxidation (anammox), in which ammonium and nitrite are converted to nitrogen gas. Hydrazine addition is known to improve the nitrogen removal capacity in anammox-based processes. However, the underlying mechanism remains unknown. This study investigated the effects of hydrazine addition on conversion performance for normal anammox substrates (ammonium, nitrite and nitrate) using batch experiments with various combinations of nitrogenous as substrates for the anammox bacteria in the presence or absence of hydrazine. Hydrazine addition improved specific anammox activity (SAA) by 42%, of which 5% was attributed to increased ammonium removal by strengthening anammox, 25% to acceleration of nitrite degradation by the bioreaction where exogenous hydrazine was reacted with nitrite to generate azide, and 12% to reducing nitrate production rate through selective inhibition of nitrite-nitrate oxidoreductase (NXR) activity by generating azide. A model in which hydrazine addition enhances the nitrogen removal capacity in an anammox system was established, which can provide theoretical guidance for engineering applications of anammox with trace hydrazine addition.