Abstract: For the underwater foundation ditch of the immersed tunnel excavated according to the plan of Nanjing immersed tunnel of high-speed railway crossing Yantze River, the head loss of the flow across the foundation ditch section is analyzed, and a high pressure district is obtained with the flow in the foundation ditch being neglected. Centrifugal model tests of soil were performed on several typical sections of the immersed tunnel under the condition of still water, namely, on the silty sand slope and the silty-fine sandy slope. The results show that the limit slope gradients of the silty sand layer and the fine sand layer is 1∶1 and is close to 1∶2 respectively, and that the damage of the underwater slope occurs at the bottom of the slope first. In addition, an elastic-plastic FEM analysis of the problem was made, with a result also supporting the conclusion that the damage of the underwater slope occurs at the bottom of the slope first.