Abstract: Decomposition of aquatic macrophyte in winter and spring is one of the important factors affecting the water quality. In order to study the suitable biomass of the declining aquatic macrophyte community in confluence area between lake and river, the decomposition experiments of the biomass of stems and leaves of remaining 0%, 20%, 40%, 60%, 80% and 100% aquatic plants after harvesting were conducted in laboratory from December 25, 2018 to April 25, 2019 in near-natural conditions. The results showed that in the decomposition process of aquatic macrophyte, the biomass of stems and leaves was the key factor affecting the nutrient concentration in water, mainly showing that the larger the biomass, the worse the physical and chemical indicators of DO, Eh and pH, and the higher the nutrient concentrations of TN and TP. The correlation analysis showed that the biomass of stems and leaves of aquatic macrophyte was significantly positive correlated with the content of total nitrogen and total phosphorus in water (P<0.01), and the correlation coefficient (R) was 0.88 and 0.83, respectively. Under near-natural conditions, aquatic macrophyte decomposed for 4 months, and the changes of nutrient content were still not stable. The remaining 20% of the biomass of stems and leaves of aquatic macrophyte after harvesting was conducive to maintaining better water quality in confluence area between lake and river.