Abstract: ［Objective］ Large-diameter caisson construction tends to induce displacement in surrounding soil, which may affect the stability and safety of nearby operational railway bridges. Therefore, it is necessary to study the impact of surrounding soil displacement caused by large-diameter caisson construction under the confinement of diaphragm walls on adjacent operational rail transit bridge deformation. ［Method］ Taking the No. 2 wall-caisson composite structure in Nantong City and the Shanghai-Suzhou-Nantong Yangtze River Bridge as research objects, field monitoring was conducted on the caisson sinking process. The variation in caisson surrounding soil displacement and its influence law on adjacent operational rail transit bridge deformation are summarized and analyzed. ［Result & Conclusion］ During the caisson sinking process, attitude inclination is prone to occur, especially in the initial sinking phase and the replacing soil excavation sinking method phase, where the inclination is particularly pronounced. The excavation and sinking of caisson may cause large-scale surrounding soil deformation. However, under the confinement of diaphragm wall retaining structure, the radial deformation of surrounding soil is significantly lower than the current 0.3% H (H as sinking depth) control limit for environmental disturbance in caisson projects. With the isolation and support provided by diaphragm wall retaining structure, the horizontal displacement and settlement of rail transit bridge piers during caisson construction can be controlled within 2.0 mm and 0.2 mm, respectively. The use of diaphragm wall retaining structure in caisson construction ensures the safety of adjacent operational rail transit bridges.