Due to the large scale, large flow velocity and complex flow structure, it is very difficult to measure the flow structure of the confluence of Yangtze River and Poyang Lake. In this study, Acoustic Doppler Current Profiler(ADCP)was used to measure the velocity and the water depth through 17 cross-sections. Kinetic energy correction coefficient(α)and momentum correction coefficient(β)were used to characterize the mixing of two flows. In addition, pH, dissolved oxygen, electrical conductivity, and suspended sand concentration were also measured. The results show that the two branches of the Yangtze River around the Guanzhou mixed quickly because of their similar flow velocity and bed discordance, while the Yangtze River and the Poyang Lake completely mix till 2. 6 km downstream where α and β are about 1. 3 and 1. 15, respectively. A 300 m stagnation area and twin surface-convergent helical cells were observed. The circulation strength of secondary flows can reach about 0. 6 and attenuates downstream. Due to decrease of width-depth ratio and effects of local wharfs, only one unidirectional counterclockwise helical cell occurred in the last two sections, and the circulation strength was lower than 0. 1.