Abstract: In recent years,the water level in Poyang Lake has repeatedly reached a record low in dry seasons,which has led to increasingly prominent economic and social development and ecological and environmental protection issues. In order to identify the law of water level changes of Poyang Lake and quantify the degree of contribution of different influencing factors on the changes in the water level of Poyang Lake, this paper uses Mann-Kendall test and polynomial regression analysis methods to quantitatively analyze the change trend of water level of Poyang Lake and its causes. In this paper, Xingzi Station and Hukou Station are taken as the water level and flow representative stations of Poyang Lake respectively, and Jiujiang Station is taken as the representative station of the mainstream of the middle and lower reaches of the Yangtze River. First, the long-term observed data of the Yangtze River Basin and Poyang Lake from 1981 to 2021 are selected for case studies. The main influencing factors of the water level changes of Xingzi Station on the Poyang Lake are identified by correlation analysis.The trend characteristics of the water level of Poyang Lake are analyzed by the Mann-Kendall test method. Finally, the degree of contribution to the changes in the water level of Poyang Lake during the dry season is quantified based on the water volume relationship. The results show that the changes in Poyang Lake′s water level from the perspective of water volume are mainly affected by the total water volume of Jiujiang Station and Hukou Station. In 1981 before the Three Gorges Reservoir was built, the lowest water level of Poyang Lake had shown a significant downward trend. After the reservoir was built, the lowest water level of Poyang Lake in the normal storage period rose by 0.44 m compared with the initial storage period, indicating that the water supply regulation of the Three Gorges Reservoir in the dry season alleviated the downward trend of the water level of Poyang Lake in the dry season to some extent. Impoundment of the Three Gorges Project is not a key factor affecting the decrease in water level in the dry season of Poyang Lake, with a contribution of 35.4%, but is mainly affected by the comprehensive factors such as decreased water and cutting under the river bed, with a contribution of 64.6%. In addition, the water supplement of the Three Gorges Reservoir has a positive effect on the increase in water level of Poyang Lake in the dry season, with a contribution of 20%, and the contribution of factors such as the cutting of the riverbed and the reduction of other incoming water is-120%. The study can provide scientific basis and guidance for formulating countermeasures for low and dry water levels of Poyang Lake in the dry season and improving water resources and water environment carrying capacity of the lake area.