In this paper, ABAQUS numerical software is used to simulate the excavation of deep foundation pit of subway in a city. The enclosing structure of the foundation pit is diaphragm wall, and the internal support is concrete support and steel support. Based on the numerical simulation, the effect of steel support on the deformation of diaphragm wall and surface settlement control was analyzed under the action of 0, 50, 60, 70, 80% preloaded axial force, and the optimal preloaded axial force value of 60% was determined. Under the optimal preloaded axial force, the steel support was analyzed for the change of support axial force and horizontal displacement, deformation of the top of diaphragm wall, maximum settlement of the ground surface, and the amount of pit bottom uplift in the process of foundation pit excavation. The results show that the support axial force and deformation, diaphragm wall deformation and surface settlement during pit excavation are within the specification requirements. The amount of the pit bottom uplift slightly exceeded the limit value, and this part of the soil should be reinforced to ensure the construction safety.