Abstract: Objective When high-speed trains operate on ballasted tracks, the complex airflow field around ballast particles can lead to track ballast splashing. The formation mechanism of this phenomenon remains unclear. It is necessary to analyse the flow field and aerodynamic characteristics around ballast particles on a ballasted track bed with surface gaps to clarify the mechanism and influencing factors of ballast splashing. Method Based on the DDES（delayed detached-eddy simulation） model, CFD（computational fluid dynamics） is adopted for numerically simulating the flow field and characteristics of aerodynamic forces around ballast particles, as the fluid flows through a ballasted track structure with bed surface gaps before and after the ballast splashing. Result ＆ Conclusion Due to the strong flow separation and fluid interaction within the ballast bed gaps, the surface flow of the ballasted track exhibits irregular development. Compared to the ballast particles on compacted ballast bed surfaces, particles located on bed surfaces with gaps are influenced by varying flow directions at the top and bottom, leading to intensified turbulent vortex development and greater pulsating forces. When a gap exists between the ballast particle and the compacted bed surface, the ground effect may cause strong flow separation and vortex shedding at the particle′s top and bottom. As the fluid passes by, the momentum transfer in the shear layer beneath the ballast particle increases, enhancing the vorticity transport. This significantly increases the lift force acting on the ballast particle, resulting in ballast splashing.