Abstract: By analyzing the seismic survey data of portal structure of highway tunnels in Wenchuan Earthquake, the results show that the structure outside the tunnels is more influenced by the secondary disasters, and impacted by seismic inertia force significantly; and that there is almost no structural damage of tunnel portal section in hard rock, while more serious damage occurs in soft rock. Through three-dimensional finite difference numerical simulation and analysis of typical field damage, seismic damage mechanism of portal structure of highway tunnels is studied. The seismic damage mechanism of structure outside the tunnels is proved, that is, the structure outside the tunnels is more easily influenced by the secondary disasters, and the seismic damage is mainly due to inertia force, and unreasonable design of wall structure and foundation of the tunnel entrance is also an important factor. The damage mechanism is proved of portal structure of tunnels. That is, when the tunnel passes through the slope with soft and hard rock surfaces directly, the tunnel structure around soft rock is greatly affected by the forced displacement. When the tunnel is covered by soft rock, the seismic inertia force is the main factor. These results are significant to the anti-seismic and damping technology of portal structure of highway tunnels.