[Objectives] To investigate the removal effect of nitrifying bacteria on ammonia nitrogen in water. [Methods] In this experiment, the treated water (referred to as raw water hereinafter) from the Changping Town Qiaoli Water Treatment Project in Dongguan City of Guangdong Province was used as the experimental water body, and the nitrifying bacteria liquid used in the project was taken as the experimental material, to explore the removal effect of the nitrifying bacteria liquid on the ammonia nitrogen in the water body. [Results] Under the condition that other variables remain unchanged, the more the amount of nitrifying bacteria liquid added, the higher the removal efficiency of nitrifying bacteria liquid; under the same conditions, the removal effect of ammonia nitrogen in a stirred water body is better than that in an unstirred water body; the removal effect of ammonia nitrogen in a water body with bio-media/carriers is better than that without bio-media/carriers. [Conclusions] Nitrifying bacteria have a better removal effect on the ammonia nitrogen in the water body