The segmental tunnel lining of existing shield tunnels is highly susceptible to crack and water leakage due to the excavation and unloading effects of adjacent foundation pits, posing significant risks to tunnel operational safety. Consequently, the effective control of crack damage is of paramount importance. This study investigates the damage and deterioration behavior of segmental linings using the total strain crack model for concrete, focusing on a subway shield tunnel located in a soft soil region adjacent to a foundation pit project. By incorporating soil-structure interaction effects, a comprehensive analysis of the damage and crack mechanisms of shield tunnels under foundation pit excavation is conducted, and the influence zones of foundation pit excavation on the damage to adjacent tunnels are delineated. The findings reveal that excavation activities induce an “oblique elliptical” deformation pattern in the segmental structure of the shield tunnel. Crack damage is primarily concentrated in the tensile zones, including the intrados of the arch crown and bottom, as well as the outside of the left and right haunches. As the horizontal distance between the tunnel and the foundation pit increases, and as the tunnel’s burial depth deepens, the location of crack damage exhibits a clockwise circumferential shift. To evaluate the crack characteristics of the segmental lining, metrics such as maximum crack width, cracking rate, and cracking factor are introduced. Based on these findings, the influence zones of foundation pit excavation on shield tunnels under various horizontal spacing and burial depth conditions are classified. Within these zones, the anticipated crack widths of the segmental linings can be estimated, providing a practical basis for implementing targeted reinforcement measures.