To investigate the grout diffusion patterns and response mechanisms during synchronous grouting for shield tunneling in sandy cobble strata, a coupled computational fluid dynamics-discrete element method model is employed to establish a numerical model of six-hole grouting. The effects of grouting-hole configuration (Type Ⅰ: arch-waist reinforced; Type Ⅱ: top-bottom reinforced), grouting flow rate (5×10－⁵－1.25×10^－4 m³/s), and grout-volume ratio (100%－250%) on grouting performance are systematically analyzed. The results demonstrate that surface settlement decreases with increasing grouting flow rate but increases with increasing grout-volume ratio. The Type Ⅱ configuration generally results in greater surface settlement than the Type Ⅰ configuration. At a low grouting flow rate (5 × 10^－5 m³/s), the grouting-hole configuration significantly affects surface settlement. However, this influence gradually diminishes as the grouting flow rate increases. Furthermore, the grout permeates and diffuses from the grouting holes into the surrounding soil, forming a semicircular diffusion zone. For the Type Ⅱ configuration and a grout-volume ratio of 200%, increasing the grouting flow rate from 5 × 10^－5 to 1.25 × 10^－4 m³/s results in a 68.5% increase in the radial diffusion distance at the tunnel crown from 52.7 to 88.8 mm. High grouting flow rates promote radial diffusion, whereas low flow rates favor circumferential convergence. Both the grout cover ratio and grout loss ratio increase with increasing grout-volume ratio. Considering filling effectiveness and cost-efficiency, the Type Ⅰ configuration is recommended for grout-volume ratios below 150%, whereas the Type Ⅱ configuration is more suitable for ratios above 200%. Variations in grouting flow rate do not significantly influence the grout cover ratio and grout loss ratio.