At present, the traffic pressure of Nanchang city, the capital of Jiangxi province, is increasing. The increasing demand for the construction of river crossing channel requires strengthening the research on the flow characteristics of the tail reach of Ganjiang River where Nanchang is located. In this paper, the south section of Nanchang city at the tail end of Ganjiang River was taken as the research object. By applying the plane two-dimensional flow mathematical model as the technical means, and using the latest measured topographic and hydrological data in 2020 to verify the model, the flow movement characteristics of the studied river section were calculated and analysed. The results show that the water surface gradient is slow, the flow field is smooth, the flow velocity is small, and the sediment is easy to deposit in this reach, which is easy to threaten the flood control of the river section.