Jiujiang hydrological station is located at the end of the middle reaches of the Yangtze River. Zhangjiazhou divides the Yangtze River into the north and south part at the 9km of the station's downstream, and Poyang Lake flows into the Yangtze River at the 30km of the station's downstream, which has a jacking effect on the water level and flow of the station, and the relationship between water level and flow is unstable. In order to improve the accuracy of both flow integration results and flood reporting, it is necessary to find the representative vertical flow velocity that can represent the average flow velocity of flow measurement sections, so that online flow monitoring can be implemented.According to the horizontal distribution of the vertical average velocity of the station and the cross-sectional maps for years, based on the measured flow results of the ADCP of the station from 2017 to 2021, the multiple linear regression analysis of the index velocity was carried out for the velocity measuring vertical lines. After analyzing the 11 schemes,there is a good relationship between the vertical average velocity and the section average velocity of the combination of two vertical lines with the starting distance of 690m and 1,110m, which is suitable to be the representative vertical line for online flow monitoring of the station．