After the 2008 Wenchuan earthquake (Mw=8.3), many mountain highway tunnels in the affected area were severely damaged. In order to improve the current knowledge about the seismic performance of mountain tunnels under strong earthquakes, a forensic investigation was conducted on 52 tunnels after the earthquake. Based on the seismic damage data, the damage level of the inspected tunnels was classified using a new damage evaluation criterion. Subsequently, typical seismic damage characteristics and mechanisms of mountain tunnels were analyzed based on three different damage patterns: damage to shallow tunnel, damage to deep-buried tunnel structure and damage to pavement. Finally, based on the relative displacement (RD) method, a three-dimensional numerical model was developed to investigate the deformation characteristics of the portal structure under a strong earthquake. The numerical results showed that the tunnel lining cross section exhibited an alternate tension–compression deformation at the diagonal directions, and the relative displacement value of the upper-structure (above the sidewall) was generally more than that of the lower-structure (under the sidewall). Three stress states of the lining were deduced from the numerical model, which provided a good interpretation for the seismic tunnel damages observed after the Wenchuan earthquake. •Forensic investigation was conducted on 52 tunnels after the Wenchuan earthquake.•Damage level of tunnels was classified using a new damage evaluation criterion.•Three typical seismic damage mechanisms of mountain tunnels were analyzed.•Three stress states of lining were deduced to interpret seismic damage of tunnel.