Tunnels are one of the most important underground structures that are being built all over the world today to develop communication routes. Given the need of society for urban and intercity tunnels to facilitate traffic, reduce traffic or shorten routes, such structures are built in different geotechnical conditions and depths. With the occurrence of devastating earthquakes in recent years and the serious damage to such structures, it has prompted mankind to turn to seismic assessment of underground structures. As an event with high uncertainty, an earthquake makes it difficult to accurately predict the amount of damage that can be caused to a structure. In this study, an attempt has been made to examine the seismic vulnerability of underground structures by considering some of the existing uncertainties. For this purpose, using the method of incremental dynamic analysis and the construction of fragility curves as a probabilistic method, the seismic vulnerability of the twin tunnels of Ahvaz Metro has been investigated. After the seismic evaluation of the studied tunnels, sensitivity analysis of parameters such as depth and stress ratio (k) has been performed and the probability of damage at each PGA level has been investigated. The results of numerical modeling and probabilistic analysis showed that in the range of 8-15 meters from the ground surface, the seismic vulnerability of the structure increases, which can be considered as the critical depth of the structure according to the characteristics of the construction site. Considering different ratios (k), it was found that the seismic vulnerability of the structure increases with the decrease in the ratio (k).