Construction of subway line intersection stations will become more and more popular in the future. Research on the deformation characteristics in the construction process is the best way to ensure the safety of the foundation pit itself, the surrounding buildings, and adjacent subway stations. In this paper, the deformation characteristics of a T-shaped subway foundation pit during construction are studied by an integrated dewatering-excavation three-dimensional numerical simulation method. The results show that the main causation of the surface settlement around the foundation pit is the stratum compression caused by dewatering, and the main causation of the deformation of the diaphragm wall and the existing subway station is the soil deformation caused by excavation unloading. It is suggested that constructing a high-quality diaphragm wall, controlling the water level within the excavation foundation pit, strengthening the monitoring, and timely cement pouring of the bottom plate are key countermeasures to reduce the deformation. The research results of this paper have reference significance for similar projects.