This paper investigates the deformation characteristics and control methods of support structures during deep foundation pit excavation, using the foundation pit at the Diejiao Station of Foshan Metro Line 3 as a case study. A 3D nonlinear finite element software is employed for excavation simulation, adopting coupled pore fluid diffusion and stress analysis. The numerical model is verified by comparing the field monitoring data with the finite element results. The study systematically analyzes the influence of reinforcement position, reinforcement parameters, and reinforcement depth on the maximum lateral displacement of the diaphragm wall of the foundation pit. The research results reveal that reinforcing the soil on both sides of the diaphragm wall can effectively reduce the lateral deformation of the foundation pit. Furthermore, the study shows that external reinforcement of the diaphragm wall can reduce the maximum lateral displacement of the wall to a certain extent, but the reinforcement effect is limited. The reinforcement effect of the inner side of the diaphragm wall improves with the increase of the excavation depth. Properly increasing the Young’s modulus of the reinforcement can reduce the adverse effect of excavation on the foundation pit. Finally, the optimal depth of reinforcement is found to be the excavation depth of the foundation pit of 28.2 m. The research results can provide a reference for the foundation pit reinforcement scheme in watery sandy soil areas.