<p>The application of reinforcing geogrid is a simple, cost-effective method for reducing permanent deformation in the ballast layer. Understanding the behavior of the ballast/geogrid system can lead to improved railway design and lower maintenance. A composite element test (CET), under simplified full-scale field conditions, was simulated by coupling discrete element method and the finite difference method. This study investigated the dynamic response and deformation behavior of geogrid-reinforced ballast under cyclic loading, focusing on variations in subgrade stiffness, geogrid location, and boundary conditions within the CET. Results indicate that greater subgrade stiffness increases the compressive force borne by the subgrade. Conversely, as subgrade stiffness decreases, the upper load is more evenly distributed to the bottom. The deployment of geogrids effectively constrains ballast particles, disperses upper loads, and reduces contact force at the model base, thereby minimizing sleeper settlement. Moreover, geogrid reinforcement is more significant for soft subgrade than for stiff subgrade. Simultaneously, the sleeper settlement under confined conditions is significantly smaller than that under unconfined condition. These results contributed to a comprehensive analysis of the mechanical properties of ballasted bed under dynamic loads, offering insights from both micro and macroperspectives. Additionally, the study clarifies the mechanism of geogrid-reinforced ballast, offering valuable insights for practical geogrid applications.</p>