To investigate the factors influencing existing tunnels adjacent to foundation pit excavations, this paper focuses on a case of excavation of MTR station foundation pits near an existing tunnel. Monitoring data are collected and analyzed, and a two-dimensional numerical model of the pit–tunnel system is established using ABAQUS (2022) software. This study examines the effect of excavation factors on the tunnel, including excavation depth, horizontal spacing between pit and tunnel, support pile wall dimensions, support structure insertion ratio, horizontal support stiffness, and geological conditions. The research results indicate that for tunnels in silty clay formations, the depth of pit excavation should be limited to 20 m. When the horizontal spacing between a foundation pit and tunnel in a silty clay formation is within 1.5 times the depth of excavation, the tunnel is more significantly affected by tunnel excavation works in the adjacent pits. Increasing the horizontal support stiffness has the greatest effect on controlling deformation and stress in the tunnel structure. The degree of influence on deformation and stress in the tunnel due to laterally adjacent excavation of a foundation pit varies with geological conditions. Among them, tunnels in muddy-silty clay and silty soils are the most affected, and it is recommended that reinforcement measures be optimized and monitoring be strengthened. The conclusions of the present study can provide a reference point and guidance for the optimal design of similar pit projects.