To ensure the structural safety of adjacent high-speed railway bridges during construction, this study systematically investigates the control effects of different retaining structures on soil deformation induced by caisson sinking. Taking the Nantong Ligang Water Plant Project, as a case study a three-dimensional finite element model was established to compare the effectiveness of three schemes: no retaining structure, planar diaphragm walls, and circular diaphragm walls. The accuracy of the model was verified using field monitoring data. Furthermore, based on the optimal circular diaphragm wall scheme, the influence of wall thickness and radius parameters was analyzed in detail. The results show that: (1) Through systematic comparison of the three schemes—no diaphragm wall, planar diaphragm wall, and circular diaphragm wall—the circular diaphragm wall was proven to be the most effective in controlling soil deformation induced by caisson construction. It significantly reduces surface settlement and horizontal displacement near bridge piers, meeting the stringent post-construction deformation requirements of high-speed railway bridges. (2) Compared with the case without a retaining structure, diaphragm walls significantly reduce ground settlement and horizontal displacement caused by caisson sinking. Under circular and planar diaphragm wall constraints, the maximum surface settlement decreased from 150 mm to 2.5 mm and 16 mm, respectively, while the radial displacement 10 m outside the caisson decreased from 32 mm to 0.5 mm and 18 mm, effectively enhancing the safety of bridge operations; (3) The diaphragm wall thickness has limited influence on the deformation of the inner sandwich soil but exhibits a significant control effect on the outer soil. As the wall thickness increases, the maximum radial deformation and the 1 mm surface settlement boundary of the outer soil show a trend of first decreasing and then increasing, with an optimal thickness of 1.2 m; (4) Within the range of diaphragm wall radii from 13.5 m to 17.5 m, larger radii significantly increase the maximum surface settlement, indicating the need for a balance between stiffness and structural stress.