Abstract: The growth and decomposition of submerged macrophytes affects the nitrogen cycle in water body. In order to investigate the migration and transformation rule of nitrogen in the water-sediment- submerged macrophytes system, a simulated system was constructed. The changes of nitrogen in water, sediment and submerged macrophytes (Vallisneria natans(Lour.)Hara, V. natans) as well as its migration and transformation in the life cycle of V. natans were studied for 270 days. The results show that the biomass of V. natans increased by more than 30 times in the first three months, and the total nitrogen (TN) amount in water and sediment decreased. After 90 days, V. natans grew slowly and stepped into decaying stage and TN of the whole system decreased gradually. TN in both water and sediment from the group with V. natans was less than that of the control group in the 180th day. In the 270th day, the decomposition rate of V. natans reached 100% and TN in sediment decreased by 9.11% compared with that in the 180th day, while the content of TN in water was similar to that of the 90th day, and was 2.42 times as that in the 180th day. NH₄⁺-N and NO₃^--N in water and sediment of the control group fluctuated slightly, whereas those in sediment and plant of the group with V. natans changed greatly. During the growth period, NH₄⁺-N and NO₃^--N in sediment transferred into V. natans, while they were released in a large amount during decomposition process of V. natans, some of which deposited into sediment.