Taking a hydropower station on the Dadu River in Southwest China as an example, a three-dimensional hydrodynamic and fish migration suitability model was established for the downstream mainstem and tributary of a hydropower station, with calibration conducted using a physical model. The study analyzed the hydraulic characteristics and fish migration suitability during power station discharge, while investigating the effects of varying inflow and generation flow rates. For tributary sections with poor fish migration suitability, different diversion wall configurations were evaluated to assess their improvement effects. The results demonstrate that hydropower discharge significantly alters hydrodynamic conditions, increasing flow velocity and water depth across a large downstream area while reducing overall fish migration suitability-effects that intensify with higher tributary inflow and generation flows. The confluence zone exhibits the most pronounced hydrodynamic impacts, with migration suitability dropping to 0.11 within a 1/6 channel-width area from the left bank under typical operation (C3 scenario: tributary inflow is 459 m3/s, generation flow is 535.0 m3/s); implementation of diversion wall scheme 7 homogenizes flow distribution and reduces highvelocity zones and recirculation, thereby markedly improving fish migration conditions.