The construction of newly built metro structures beneath existing ones has a pronounced influence on the deformation behavior and safety of existing metro structures. However, most existing studies concentrate on individual case projects, with limited systematic or generalized insights. To address this issue, the authors investigate 18 representative undercrossing projects in Beijing through a multi-case synthesis and comparative analysis. Project backgrounds and settlement monitoring data are compiled to trace the evolution of deformation control measures for existing structures. Based on their control targets, these measures are classified into three categories: controlling deformation sources, transmission paths, and sensitive structures. Considering deformation characteristics, existing structures are further categorized into three types—single tunnel section, segmental tunnel lining, and multiple tunnel sections—and their settlement patterns are comparatively analyzed. Results show that the settlement curves of single tunnel section and tunnel lining type can be well described by Gaussian functions, while multiple tunnel sections exhibit distinct patterns due to joint deformation, with the single tunnel section type showing the best fitting performance. Based on these findings, two safety evaluation indicators are proposed, among which the maximum tensile strain derived from Gaussian fitting provides a more effective measure of the stress state and safety condition of existing structures.