<p>Frequent leakage induced by lining cracks in metro tunnels poses a significant threat to operational safety. This study investigates crack-induced leakage in 22 metro tunnels excavated by the mining method in Beijing using field inspections and numerical simulations. Field data were analysed by combining quantitative parameters (crack width and length) with qualitative descriptors (crack location and pattern) to examine their relationships with leakage level. Pearson correlation and analysis of variance results indicate that crack width exhibits a strong and statistically significant correlation with leakage level, whereas crack length, location, and crack pattern show limited influence. A three-dimensional lining–crack coupled numerical model was developed to simulate leakage evolution under varying crack widths and groundwater heights. Numerical results show that leakage is strongly concentrated within crack zones due to the pronounced permeability contrast between cracks and intact lining. For a given crack width, the leakage volume per unit length (<i>Q</i>_M) increases approximately linearly with groundwater height within the investigated range. The leakage formation period decreases approximately exponentially with increasing crack width and rising groundwater head, allowing leakage evolution to be classified into four temporal regimes from transient to lagged. Leakage velocity increases monotonically with crack width and can be categorized into three stages from low to high intensity. Based on numerical results, empirical <i>Q</i>_M–<i>d</i>_f–<i>H</i> relationships were established and calibrated for representative groundwater conditions in the Beijing metro system. The proposed field-numerical-empirical framework provides a practical and quantitative basis for evaluating crack-induced leakage level in mining-method metro tunnels.</p>