Abstract: NH₃-N is the main nitrogen pollutant in the slightly contaminated water. Comparative analyses on nitrogen removal efficiency and nitrification/denitrification intensity were carried out to indicate the nitrogen removal process in the treatment of slightly contaminated water by the horizontal subsurface flow constructed wetlands (HSCWs), and PCR-DGGE was conducted to detect the effects of plants and NH₃-N on community structure of ammonia-oxidizing bacteria (AOB) in HSCWs. The results showed that when the NH₃-N concentration was 0.6-0.7and 4.5-5.0mg/L, and the hydraulic retention time (HRT) was 2.5d, TN removal efficiency of reed HSCWs reached 81.9% and 62.2% respectively. Moreover, the ammonium content and reeds significantly improved the nitrification intensity and AOB community structure in the wetland. When the polluted water with high and low concentrations of NH₃-N was treated by HSCWs for 50days, the nitrification intensity were 0.164and 0.103mg/(kg·h) respectively, and the Shannon-Weaver diversity index of AOB community were 2.32and 1.75when the process were for 90days. When raw water was polluted water with a high concentration of NH₃-N,as for the wetland planted with reeds, at 50days, the nitrification intensity increased from 0.082to 0.164mg/(kg·h), and at 90days the Shannon-Weaver diversity index of AOB community increased from 1.95to 2.32.