Abstract: In order to explore the migration of nutrients released by decomposition of aquatic plants in the sediment-water-macrophyte system and determine the optimal biomass of aquatic plants after harvest, the decomposition process of mixed aquatic macrophytes under different biomass gradients was studied in a plastic ventilated greenhouse. The aquatic plants in the Lihu Lake-Ludianqiaobang River confluence area in winter were selected as the research objects. Based on the actual harvest experience, the decomposition experiment biomass was set to 0%, 20%, 40%, 60%, 80% and 100% of the total biomass except roots. The experiment was conducted on December 25^th, 2018 for a total of 150 days. The experimental results showed that: (1) Compared with the decomposition experiment in a constant temperature chamber, the first two stages of decomposition of mixed aquatic plants under near natural conditions were long-term and persistent. (2) The decomposition of aquatic macrophytes resulted in a rapid increase in C, N and P contents within 30 days, reaching a peak on about the 70th days, and slowly decreasing to a stable state after 100 days. The whole change process lasted for nearly 120 days, but the macrophyte stems and leaves were not completely decomposed, and most of them were deposited on the surface of the bottom sediment. (3) In the sediment-water-macrophytes system, the sediment was mainly released in the initial stage of the experiment, and was mainly stored by adsorption after 30 days. Correlation analysis showed that the biomass of macrophyte stems and leaves was positively correlated with the nutrient concentration in water and sediment. Among the 6 experimental groups, the group with 20% of the aquatic plant biomass remains after harvest was beneficial to the water quality of lake-river confluence area.