According to the disaster characteristics of mountain tunnels in loess areas under strong earthquake, the ground motion amplification effect of the soil around tunnel portal and the tunnel structure were analyzed. Through a 3D numerical model, the acceleration and displacement responses in the slope elevation direction and horizontal direction of the model with different slope, slope height, and portal elevation were mainly studied, and the responses of lining structure were also analyzed. The range of slope reinforcement area and the recommended seismic fortification length of tunnel were proposed. The research results showed that the changes of elevation, slope angle, and portal elevation all have an important impact on the seismic response law and failure mode of the tunnel portal section. The shear failure at the bottom of slope is easy to occur on gentle slope, while the tensile failure at the top of slope is easy to occur on steep slope. With the increase in slope height, the sliding failure range of the slope increases gradually. The existence of tunnel has a significant " restraining" effect on the amplification effect of slope elevation. There is a dynamic response amplification area in the horizontal direction of the portal, ranging from 2.1 to 2.8 times of the tunnel diameter. Based on the dynamic response law of lining along the tunnel length, it is suggested that the minimum seismic fortification length of the portal section of loess tunnel is 3 times of tunnel diameter.