Abstract: The nutrient release from submerged macrophytes (Vallisneria spiralis) during the decomposition process and its effects on water quality were studied in this study. The situation of water, sediment and plants in Taihu Lake Taihus was simulated and the mechanism was revealed by planting the macrophytes in the greenhouse. Under the experimental condition with the biomass of 689g/m², the decomposition of V. spiralis was conducted at two stages. The first stage was from October to next February (autumn and winter) with low temperature, V. spiralis decayed day by day. During this stage, not much nutrients were released to the water and the sediment and most of the nutrients were kept in the remains of the plant. Meanwhile, pH and the dissolved oxygen concentration had no change. The second stage was from March to April when the weather was warm, the decaying of plants became faster and a lot of nutrients were released to water and sediments, and most of them came into sediments. From February to March, concentrations of TOC, TN and TP increased by 216.64%, 60.96% and 144.40%, respectively; pH increased 6.27%, dissolved oxygen concentration dropped by 91.5%. The different nutrients transformed and they were transferred among water-sediment-plant when the submerged macrophytes decayed.