Changes of cushion roughness caused by scraping-paving and change of particle size will affect slurry diffusion, grouting volume and grouting pressure, when adopting grouting method in foundation treatment of immersed tunnel. Model test was conducted to analyze slurry diffusion, grouting volume and grouting pressure during single-hole grouting and dual-hole grouting, adopting 2mm, 5mm and 10mm diameter particles for cushion. It was indicated that the velocity of slurry diffusion and grouting volume increased with the increase of cushion particle size. The range of grouting pressure fluctuation would be larger when adopting larger particle size cushion, the grouting pump would be difficult to control and the slurry diffusion would turn to one direction. Based on the test result, the slurry diffusion velocity, grouting volume and grouting pressure ought to be considered comprehensively when choosing cushion particle size.