A theoretical analysis method for shield tunnel lateral deformation considering factors such as the deformation of the foundation pit enclosure structure, initial state of the tunnel, size effects, and longitudinal actions is proposed. This method aims to explore the response of side excavation to the lateral deformation of existing shield tunnels. The study examines the influence of tunnel section size, burial depth, and horizontal net distance from the enclosure structure on tunnel lateral deformation. Based on a deep foundation pit project near the Zhijiang Canal tunnel, the theoretical solution is validated through field measurements and numerical analysis. The results reveal the distribution pattern of additional stress in the soil outside the foundation pit and the response law of tunnel lateral deformation. The results reveal the following: (1) The lateral deformation of the tunnel caused by side excavation increases approximately in a parabolic shape with increasing tunnel section size. (2) When the burial depth of the tunnel′s central axis exceeds 2.5 times the excavation depth of the foundation pit, the horizontal clearance slightly affects the lateral deformation of the adjacent tunnel. (3) When the horizontal clearance between two tunnels exceeds 40 m, the burial depth of the tunnel has a relatively small impact on the lateral deformation of the adjacent tunnel. (4) The calculation results effectively reflect changes in lateral deformation and ellipticity of the tunnel caused by side excavation. This provides a new approach to predicting deformation, internal forces, and defect development in tunnel structures owing to excavation, allowing for the dynamic regulation of foundation pit construction and ensuring the operational safety of existing tunnels.