Abstract: A pilot-scale subsurface flow constructed wetland (CW) was built to remove nitrogen from pre-treated domestic sewage. The planar distribution characteristics of different nitrogen forms within the CW and the composition of different nitrogen forms in inflow/outflow were investigated tostudy the nitrogen removal mechanism, and the nitrogen removal efficiency was calculated. In the experiment, the inflow had the following concentrations: TN of30-115 mg/L, NH₄⁺-N of 25-60 mg/L, COD_Cr of 90-190 mg/L and hydraulic retention time of 4 d. The removal rates of TN and NH₄⁺-N were32%-70% and 33%-73%, respectively. Nitrification/denitrification processes are themain mechanisms for nitrogen removal in the CW; nitrification mainly happens inthe shallow part of the system and denitrification in the deep part. In the deeppart, NH₄⁺-N removal is limited. To improve the nitrogen removal efficiency of the tested wetland, DO content should be enhanced and the process of mineralization of organic nitrogen should be strengthened.