Abstract: Ship-lock emptying is a critical unsteady hydraulic process for inland navigation hubs. For double-lane ship locks sharing a single lower approach channel, synchronous water discharge tends to excite complex adverse flow phenomena, including reciprocating flow, large-scale backflow, and oblique cross-flow within the navigation water area. These flow disturbances produce high local flow velocities together with substantial water-surface fluctuations along the channel. As a consequence, moored vessels may suffer excessive mooring-line tension, and moving ships are exposed to risks of drift, yaw and collision, which seriously jeopardize navigation efficiency and vessel safety. Targeting the engineering problem of poor navigable flow conditions and excessive flow velocities in the shared lower approach channel of the Mujing double-lane ship lock, this paper uses a three-dimensional hydrodynamic numerical approach to investigate feasible discharge optimization strategies. The FLOW-3D computational fluid dynamics software is employed, and the RNG k-ε turbulence model is selected to close the Reynolds-Averaged Navier-Stokes equations for capturing turbulent mixing and energy dissipation during lock discharge. The computational domain covers the lock discharge outlets, the full curved lower approach channel, the berthing segment, the main river channel and adjacent riparian water zones. Grid-independence tests are performed with three sets of meshes; finally, 5.5-million hexahedral structured meshes with local refinement around outlets, channel side-walls and the berthing zone are chosen to balance computational accuracy and computational cost. The numerical model is further validated against physical-model test data of analogous ship-lock filling-emptying system. The simulated unsteady water-level evolution inside the lock chamber agrees well with experimental records, and minor discrepancies are mainly attributed to scale effects in physical model experiments, which confirms the reliability of the established numerical framework. Multiple simulation cases are designed to quantitatively compare hydraulic responses under varied valve-opening durations and different discharge layout alternatives. Key hydraulic indicators, such as discharge hydrograph, spatial velocity distribution, instantaneous flow patterns, water-surface gradient and transverse velocity within the berthing section, are extracted and analyzed in detail. Under the simultaneous-discharge condition of double-lane ship locks, as the valve-opening duration is extended from 5 min to 7 min, the peak discharge rate decreases gradually from 329.3 m³/s to 295.2 m³/s, and the time required to attain peak discharge is delayed from 238 s to 302 s. Nevertheless, even with prolonged discharge time, all three cases with 5-min, 6-min and 7-min valve-opening still exhibit extensive regions where flow velocity exceeds the threshold of 1.0 m/s stipulated by ship-lock design specifications. Such excessive velocities persist in both the curved channel segment and the berthing zone, and cannot satisfy the safety requirements for ship navigation and berthing. The maximum water-surface gradients measured in the berthing section reach 1.7‰, 1.4‰ and 1.2‰ for 5-min, 6-min and 7-min discharge cases, respectively. Merely optimizing the outlet structure, including modifying grille arrangement, adding energy-dissipation sills and constructing stilling basins, yields only marginal improvements in the flow field. Large high-velocity zones still remain in the approach channel, demonstrating that outlet-only reconstruction is insufficient to resolve the hydraulic defects. Accordingly, an integrated optimization scheme combining outlet structural improvement and side-discharge technology is proposed. By diverting a portion of the lock discharge directly into the main river through side galleries instead of releasing all flow into the narrow approach channel, the flow intensity entering the navigation channel is effectively reduced. After implementing this combined optimization measure, the maximum longitudinal flow velocity in the berthing section drops to approximately 0.95 m/s; the maximum water-surface gradient is reduced to 0.80‰, and transverse flow velocity remains relatively low. The original disordered flow patterns are greatly suppressed, the flow regime in the whole lower approach channel becomes far more stable, and the overall navigable flow conditions are significantly improved. This research demonstrates that the combined scheme of outlet optimization and side discharge can effectively mitigate the adverse hydraulic impacts induced by synchronous emptying of double-lane locks sharing one lower approach channel. The quantitative research outcomes provide practical and valuable technical references for discharge-layout design and navigable-hydraulic-condition optimization of similar inland hydraulic-hub ship-lock projects.