Abstract: Six kinds of aquatic plants commonly used in constructed wetlands, namely Canna indica, Acorus calamus, Typha orientalis Presl, Phragmites communis, Eichhornia crassipes and Nymphaea tetragona, were selected for nitrogen tolerance analysis. They were cultured in nutrient solutions of different ammonia nitrogen concentrations. The main enzyme indexes of plant antioxidant enzyme defense system were tested in a certain period of time. The nitrogen tolerance of six species of aquatic plants was preliminarily found by measuring the content of malondialdehyde (MDA), the activities of superoxide dismutase (SOD) and catalase (CAT) in leaves. The results showed that the SOD of Acorus calamus reached the peak when the concentration was about 300 mg/L, then decreased. The SOD of Eichhornia crassipes, Typha orientalis Presl and Nymphaea tetragona reached the peak when the concentrations were around 500 mg/L. The SOD of Canna indica and Phragmites communis reached the peak when the concentrations were over 600 mg/L, and showed a steady growth trend in the early stage. The CAT of Eichhornia crassipes, Nymphaea tetragona, Typha orientalis Presl and Acorus calamus reached the peak when the concentrations were all about 500 mg/L, and showed a steady growth trend in the early stage, while the peak values of Canna indica and Phragmites communis were between 600~700 mg/L, in which Phragmites communis increased steadily, and Canna indica decreased at first and then increased. The MDA of Eichhornia crassipes and Nymphaea tetragona reached the peak when the concentrations were around 300 mg/L. The MDA of Typha orientalis Presl and Acorus calamus reached the peak when the concentrations were around 500 mg/L. The MDA of Canna indica and Phragmites communis reached the peak when the concentrations were over 600 mg/L. The results showed that Acorus calamus, Eichhornia crassipes, Phragmites communis and Nymphaea tetragona were suitable for low concentration ammonia nitrogen environment. Although Canna indica and Phragmites communis were subjected to some stress of low concentration ammonia nitrogen, they performed well at high concentration, and Canna indica and Phragmites communis were more tolerant to ammonia nitrogen.