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In soft-soil areas, excessive transverse deformations have occurred in numerous shield tunnel linings accompanied by serious structural defects. However, to date, a method to effectively diagnose the health status of these poor-performing structures is unknown. This study focuses on health diagnosis for the typical stagger-joint-assembled shield tunnel linings in soft-soil areas of China. First, based on theoretical analyses, it is assumed that diametral deformation can be a significant indicator for the structural performance of a lining; subsequently, this is verified by abundant field data. In particular, a strong positive correlation between the diametral deformation and development of defects is observed. Subsequently, a full-scale test is performed to reveal further quantitative correlations. Besides the service limit values determined in the range of 0.24%–0.79% by specifications and prior studies, the test reveals detailed structural performance deterioration when the deformation is getting larger, where 1.5% is considered a threshold for the failure the entire structure. Additionally, the values of 0.5%, 0.8%, and 1.1% are observed to be key points indicating the gradual degradation of waterproofing, cracks, and overall mechanical performance of the lining. Based on these key points, a five-level health-degree model is constructed to determine the health status of a ring that can be regarded as a supplement of the previous limit values, which is more detailed and practical for the common large-deformed in-service linings in soft-soil areas. These findings can also serve as benchmarks for related studies on linings with different structural parameters in the future. [ABSTRACT FROM AUTHOR]