Abstract: Shield tunnel grouting is important to guarantee the settlement of the tunnel as well as to slow down the ground surface settlement, however, most of the grouting during shield construction lacks a complete and systematic theoretical, especially its numerical realization, which is rarely carried out. For backfill grouting during shield construction, a numerical simulation based on DEM-FDM coupling and particle inlet is carried out to realize a refined simulation of slurry injection from the grouting hole into the shield tail gap and generate slurry pressure on the surrounding rock and pipe sheet. The diffusion state of the slurry in the gap behind the pipe sheet and the crowding effect on the surrounding rock for the two forms of shield tail gap are studied and analyzed. The results show that the diffusion of slurry in the shield tail gap can be divided into three stages: the diffusion stage with each grouting hole as the starting point, the interconnection and contact stage of the slurry bubbles injected into each grouting hole, and the final gap filling stage; during the grouting process, the slurry is injected into the shield tail gap at a certain initial velocity, but the slurry diffuses slowly to both sides and upward, thus causing a rapid increase in stress, and when the grouting is stopped, the The slurry pressure gradually dissipates and stabilizes, showing a decreasing trend; the slurry injected into the shield tail gap can significantly reduce the ground surface settlement.