<p>Debris flows are among the most destructive geological hazards, and barrier systems are widely used to retain bedload and dissipate their energy. This study investigates the performance of barriers with different parameters (height, location, and multi-stage deployment) during the transition of debris flows from a stable bed to an erodible bed, and provides guidance for barrier design. SPH-DEM-FEM coupling is employed to gain insight into bedload retention and energy dissipation in flow–barrier–bed interactions. Material and energy criteria are proposed to evaluate the performance of barriers. Force-chain analysis and the criterion results show that barriers significantly alter the bed’s boundary conditions, enhancing the shear resistance of the bed particles and improving material-trapping efficiency. Moreover, strengthening two key interactions (debris flows–dead zone, and debris flows–downstream channel bed) is essential for improving energy dissipation. Higher barriers improve bedload retention and energy dissipation but cannot be raised indefinitely because of induced erosion downstream. Barriers located upstream or downstream achieve higher material and energy criteria, with the upstream one recommended for producing a shallower erosion pit. A multi-stage barrier retains more bedload than a single-stage one, but its energy dissipation improvement is limited. Therefore, additional measures are needed to enhance the energy dissipation efficiency of multi-stage barriers. The in-depth analysis of the barrier bedload retention and energy dissipation supports the effective design of barriers for mitigating debris-flow hazards.</p>