<p>Multi-stage check dams represent an effective approach for incrementally mitigating debris flow impact. However, current methodologies for calculating overflow distance exhibit certain limitations, and the combined influence of key design parameters on impact forces remains poorly understood. This study introduces a novel overflow distance calculation method grounded in the principle of energy conservation. A coupled numerical approach combining Smooth Particle Hydrodynamics, the Discrete Element Method, and the Finite Element Method (SPH-DEM-FEM) was employed to simulate debris flow dynamics and optimize check dam configurations. Results demonstrate the superior accuracy of the proposed overflow distance model. It is recommended that dam spacing in straight gullies should not exceed twice the overflow distance. Optimal design is achieved when the normalized height of the first dam does not exceed 8 and the heights of the two dams are equal. Furthermore, an empty reservoir is not necessarily the most hazardous condition, as the effect of siltation depends on both the silt material type and filling level.</p>