Fluid-solid coupling problems in geological engineering systems are of significant importance, as deformation, instability, and seepage phenomena frequently occur in natural soil and rock masses. Traditional research methods primarily rely on theoretical analysis and physical experiments; however, these approaches often struggle to accurately capture the complex interactions between solid particles and fluid flow. In recent years, advancements in numerical simulation methods, particularly the integration of the Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD), have provided a powerful framework for analyzing coupled fluid-solid processes. The DEM-CFD method using the Lagrangian-Eulerian coordinate system enables synchronized simulation of particle motion and fluid dynamics, thereby more accurately depicting particle-fluid interactions. This study aims to explore collapse mechanisms and seepage-induced deformation in backfill soil using the DEM-CFD coupled method. By establishing a three-dimensional numerical model and analyzing the evolution of particle migration, loss, and settlement, we identify critical failure thresholds and assess the influence of seepage radius. Additionally, a two-dimensional model is employed to further investigate the impact of particle flow on backfill stability. These results can be used to predict seepage failure and enhance the safety of geotechnical engineering structures.