Abstract: The Neijiang Reach of the Dujiangyan is a typical bifurcated channel, characterized by complex and variable flow processes, with flow patterns directly influencing the water diversion efficiency and flood control safety of the Chengdu Plain. To systematically reveal the hydraulic characteristics of this reach and evaluate the impact of the fixed-bed dam project, T a two-dimensional hydrodynamic model validated by experimental data was employed to simulate and analyze the flow characteristics under various inflow conditions during different periods. The results show that: (1) the construction of the fixed-bed dam significantly altered the local flow structure in the Neijiang Reach. The backwater and flow-guiding effects of the dam caused more flow to be discharged through the Feisha Weir under high-flow conditions, leading to increased flow velocity and decreased water level at the cross-section upstream of the dam. (2) influenced by the flow-constraining effects of the Lidui and Renzidi, three planar vortices (two counterclockwise and one clockwise) formed at the front of Baopingkou. The sizes of the Wotie Vortex and the Lidui Front Vortex showed a clear negative correlation with the inflow rate. (3) Under high-flow conditions, the flow diversion effects of the Feisha Weir and the spillway were significant, reducing the diversion ration of the Baopingkou from 91% to 43%. The construction of the fixed-bed dam further optimized the flow distribution pattern, resulting in a notable decrease in the diversion ration of the Baopingkou and significantly enhancing the flood control safety of the Chengdu Plain. The findings of this study can provide theoretical support and engineering references for the efficient utilization of water resources and flood control regulation in the Dujiangyan Irrigation System.