The north canal, which runs through the Beijing-Tianjin-Hebei Region, is the focus of the improvement of the Haihe River Basin during the 13th five-year plan period and will be followed by a water purification demonstration project. By monitoring the concentration of ammonia nitrogen, total phosphorus and COD in the water of the north canal city section, the spatial and temporal characteristics of water quality in the north canal city section are analyzed. By taking the river water of north canal as raw water, the experiment is carried out to study the removal efficiency of ammonia nitrogen, total phosphorus and COD in the raw water of north canal under different aquatic plants, different coverage area ratio and different hydraulic residence time. the dominant plant species - water onion is selected. The removal rate and change rule of ammonia nitrogen, total phosphorus and COD are obtained under the conditions of coverage area proportion of 25%, 50% and 67%. When the hydraulic residence time is 24 h, the removal rate of ammonia nitrogen, total phosphorus and COD in the water body of the north canal can reach 39.5%, 65.2% and 45.9% with the proportion of 25% covered area, and the hydraulic load parameters are 0.49, 0.15 and 17 mg/(L·d), respectively. The results are good and the demonstration project cost can be taken into account.