The network layering of a tunnel lining drainage system is conducted, and the initial drainage state indicators and calculation methods of its drainage function threshold and usage rate at the node, pipe section, and unit level in a unit network are proposed to quantify the impact of the tunnel lining drainage system and its diseases, and effectively evaluate its drainage status. Further, the typical disease forms, such as leakage and blockage, are summarized. The betweenness and number of accesses are defined to represent the structure and function importance as the weight calculation of the unit network, quantitative influence, and node disease calculation on the drainage function threshold, utilization rate, and comprehensive loss rate of the pipe section and unit are discussed, and calculation examples are designed to verify the results. The calculation example results show that the comprehensive drainage state index of vertical pipes is 0.001 6, which is lower than 0.002 3 of the horizontal pipes, and the unit comprehensive drainage state index, s, is 0.040 3, indicating that the unit drainage state used as a quantitative indicator of the drainage behavior of the lining drainage system can effectively realize quantitative evaluation of the lining drainage system.