Abstract: Neck cutoff as an extreme geomorphic process, occurred in flood season of July 2018 in the lower Black River in the Yellow River Source region. Neck cutoff strongly adjusting flow structure is of great importance to understand the hydrodynamic mechanism of meander cutoff. Acoustic Doppler Current Profiler (ADCP) was used to conduct field measurements on a total of 45 cross-sections in May 2019 and August 2020 in order to compare and analyze three-dimensional flow structure in different cross sections. Results demonstrated that due to the influence of channel shape, boundary conditions, hydrological conditions and other factors, the flow structure in the upstream straight reach is less affected by the neck cutoff, while the flow structure of the bend section is obviously adjusted after neck cutoff. The specific performance is that the neck cutoff has little effect on the spatial distribution of velocity and circulation intensity in the straight reach. It will change the transverse distribution of the maximum velocity at the top of the bend and the section distribution of circulation structure, and affect the scale and distribution position of the separation area. The results also deepen the understanding of the impact of flow structure adjustment after neck cutoff on deposition of oxbow lake, scouring and depositing of riverbed in separation area and scouring of new channel.