Abstract: The collapsible deformation characteristics of loess foundation after flooding are of great significance in the engineering construction in loess regions. However, previous studies of the collapsible characteristics of deep loess foundation soil during the process of flooding and the variation law of soil pressure are not perfect. In this paper, a site in the South Station of Lanzhou New District of the Zhongwei-Lanzhou Railway is taken as an example and a field immersion test is carried out at the site to monitor the surface displacement and deep wet sink deformation of the foundation of the deep wet sink loess site. At the same time, soil pressure boxes are buried to analyze the change law of vertical soil pressure of the foundation soil during the immersion wet sink process. The research results show that (1) the maximum wet sink of the deep loess site is around 150 cm, and wet sink basically does not occur at the ground surface outside 10 m of the water immersion range. (2) According to the deep settlement test results, it can be seen that the loess foundation 0−15 m deep is the range of the strong wet sink soil layer, and the wet sink amount reaches about 120 cm, accounting for more than 80% of the wet sink amount of the whole site. Wet sink does not occur in the foundation soil 24 m deep and it is a non-self-weight wet sink soil layer. (3) According to the soil pressure monitoring analysis, it is obtained that the soil structure within 15 m in depth is completely collapsed after water immersion, and the soil pressure obtained from the test is the self-weight pressure of the overlying soil. A certain amount of wet subsidence after water immersion occurs at a depth of 15 m or less, but the soil structure is not completely destroyed, and it still has a certain support effect on the overlying self-weight load. The results of this study can provide some technical references for the later construction of the Zhongwei-Lanzhou Railway and the Lanzhou New District.