Abstract: To study the mechanism of horizontal subsurface constructed wetland, three full scale constructed wetland used to treat black-odor river were studied on pH, + 4 NH -N, NO2? -N, NO3? -N and the number of ammonia oxidizing bacteria and nitrite oxidizing bacteria. Shaking conical flasks method was used to observe the transformation of + 4 NH -N under the aerobic condition. It was found that pH was alkalescence and changed slightly along with the length of constructed wetland. Dissolved oxygen concentration ranged from 0.09 mg/L to 0.35 mg/L. The concentrations of ammonia nitrogen were characterized by a trend of decrease along with the length of all CWs. NO3? -N and NO2? -N was always low along with the length of constructed wetlands. There were more ammonia oxidizing bacteria than nitrite oxidizing bacteria in LDP constructed wetland, whereas, the number of ammonia oxidizing bacteria was approximately equal to the number of nitrite oxidizing bacteria. In onsite artificial aeration system, it was found that partial nitrification and denitrification occurred in the CWS. Approximately 50% nitrogen was removed by partial nitrification and denitrification in two Lao duanpu constructed wetlands, and 20% in Beixia constructed wetland.