Abstract: The constructed wetlands are widely used in deep purification of tailwater from sewage treatment plants due to its advantages in low investment, simple operation, and low-cost operation. Based on the continuous monitoring on the nitrogen and phosphorus in the water body and sediment in a constructed wetland ecosystem (consisting of a subsurface flow wetland and eight plant ponds successively) for purifying the tailwater from the Xingning Sewage Treatment Plant in the Dongjiang Lake, this study analyzes the purification effect of nitrogen and phosphorus in water body and its relationships with nitrogen and phosphorus in the sediment. The constructed wetland ecosystem has obvious efficiency on the removal of nitrogen and phosphorus from tailwater. The efficiency on removal of total nitrogen, ammonia nitrogen, nitrate nitrogen, and total phosphorus in the water body were 62.6%, 45.1%, 58.3%, and 47.7%, respectively. Compared with the efficiency on the removal of nitrogen and phosphorus from tailwater in the subsurface flow wetland located at the front of the wetland, the efficiency in the plant ponds located at the back was relatively low. The efficiency on removal of nitrogen and phosphorus in the water body varied significantly among the plant ponds. Generally, the ponds with larger areas showed higher efficiency on removal of nitrogen and phosphorus, whereas the ponds with smaller areas showed lower efficiency. However, the efficiency on removal of nitrogen and phosphorus in some ponds was less than zero, indicating the nitrogen and phosphorus were released into water in the ponds. Additionally, the concentrations of total nitrogen, ammonia nitrogen, nitrate nitrogen, and total phosphorus in the sediment of the constructed wetland showed a decreasing trend with the direction of water flow and had a significant positive correlation with the concentrations and the removal efficiency of nitrogen and phosphorus in the water body. The contribution percentage of total nitrogen in sediment to the efficiency on the removal of nitrogen and phosphorus from tailwater was 33.4%, and the contribution percentage of total phosphorus in sediment to the efficiency on the removal of nitrogen and phosphorus from tailwater was 32.5%. It is clear that the effects on the purification of nitrogen and phosphorus in the constructed wetland are associated with the types of constructed wetlands, areas of plant ponds, and the concentrations of nitrogen and phosphorus in the sediment. Consequently, the recommendations for optimization of the environmental conditions and adjustment of the biological structure are proposed for reconstruction of the subsurface flow wetland and plant ponds with the aim to provide the reference for constructed wetlands management.