When tunnels are built in saturated silty sand, the tunnel leakage can carry fine particles into tunnels and generate seepage erosion process. During this process sand particles are subjected to high confining and hydraulic pressures and then are eroded through the seams of segmental joints. This paper investigates the mechanism of seepage erosion process using Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) coupling simulations. The seepage erosion processes are simulated for loose, medium dense and dense silty sand, respectively. The evolution of the fine particles loss and the volumetric strain are investigated. Results show that the fine particles are eroded in two patterns. The first pattern is induced by axial pressure extruding fine particles through seams without hydraulic pressure. The second pattern is induced by fluid drag force dragging fine particles under hydraulic pressure. Correspondingly, the erosion process is divided into two stages as initial extruding stage and the following eroding stage. Result shows that dense sand is more prone to particle erosion in the first pattern while loose sand are gradually more prone to particle erosion in the second pattern. The quantitative relationship between the fine particles loss, the volumetric strain and the four influencing factors (i.e. time, hydraulic pressure, consolidated stress ratio and void ratio) are investigated using regression analysis based on 81 numerical simulations, respectively. The flow paths of the eroded fine particles are also investigated during the erosion process, which demonstrates that flow paths change alternatively between the blocked state and the opening state and then more flow paths in the model will open as the erosion process carries on.