The change of the relationship between rivers and lakes affected by the Three Gorges Reservoir has been a major focus in recent years. The backflows from the Yangtze River to the Poyang Lake is the main feature of the relationship, and its variation characteristics and occurrence conditions are still controversial. The hydrological data since 2003 of the Jiujiang, Balijiang and Datong hydrological station located in the Yangtze River and the Duchang, Kangshan and Hukou hydrological station located in the Poyang Lake were used. The relationship between the Yangtze River and the Poyang Lake after the operation of the Three Gorges Reservoir was analyzed from the aspects of water level, flow rate, inter-annual variation and annual distribution of runoff. The internal mechanism of the backflows from the Yangtze River to the Poyang Lake based on the basic law of energy balance was discussed and the theoretical formulas were deduced. The results showed that: 1) Poyang Lake had the characteristics of both river channel and lake. The characteristics transforms from river channel to Lake when the water level of Duchang station was above 15 m. The lake characteristics was presented when the water level of Kangshan station was above 17 m;2) The backflows from the Yangtze River to the Poyang Lake occued mainly in the flood season from July to September. In the total days of backflows, 41.6%,24.8% and 28.8% belonged to July, August and September respectively. The intensity of backflows was weakened to a certain extent by the flood control operation after the 175 m deep experimental impoundment of the Three Gorges Reservoir.3) The conditions of water level of Yangtze River were higher than that of Poyang Lake, large floods in the Yangtze River, and total inflow of the Poyang Lake receding or remaining stable were necessary for the formation of backflows. Furthermore, the magnitude and duration of the backflows mainly depended on the Yangtze River flood peak. The proposed methods and conclusions can provide a reference for further understanding the relationship between rivers and lakes and their interaction with each other,and provide scientific evidence for the comprehensive management of the Poyang Lake.