To investigate the deformation induced by deep foundation pit excavation sharing a diaphragm wall with an existing subway station, this study took the foundation pit project of Gongming Square Station of Shenzhen Metro as an example. A three-dimensional numerical model was established by FLAC^3D software to analyze the deformation characteristics of the surrounding ground surface, retaining structures, and the subway station during the excavation process. In addition, field monitoring was conducted to measure the vertical and horizontal displacements of the pit sidewall and the subway station sharing the diaphragm wall. The results show that after excavation, the ground surface settlement behind the retaining wall exhibits an approximately elliptical distribution, with the affected range being about one time the excavation depth. The retaining wall shows lateral deformation toward the pit, forming an “inward convex” shape. The excavation mainly causes uplift displacement in the subway station sharing the diaphragm wall, while lateral displacement remains small. The maximum uplift displacement (2.85 mm) occurs at the center of the sidewall adjacent to the pit, and the maximum lateral displacement (1.44 mm) occurs at the base slab near the station center. The field monitoring data of the pit sidewall and subway station sharing the diaphragm wall agree well with the numerical simulation results, verifying the reliability of the numerical model.