Chute cutoff in Xiongjiazhou reach in the tail of lower Jingjiang River occurred in 1950 and gave rise to a new cutoff channel, resulting in the adjustment of downstream hydrodynamic condition in the Qigongling Bend, which further aggravated bank collapse and weakened channel stability. Based on hydrological data and channel topography, a two-dimensional hydrodynamic model of the Xiongjiazhou reach (Jianli-Luoshan) was constructed using MIKE21 to simulate the influence of the cutoff channel development on the hydrodynamic adjustment under different discharges and branch sizes. Results manifested that widening the branch leads to the increasing of flow diversion, and the flow dynamic axis deflected along the concave bank of Qigongling Bend. Given that the branch size enlarged, the outflow of the diversion flow was roughly perpendicular to the upstream bank at the neck of Qigongling Bend, which aggravated bank collapse of the unprotected river bank on the upstream side. The result was confirmed by in situ field observation.