Based on shaking table tests and multi-index analysis incorporating Arias intensity and wavelet packet energy spectrum, this study elucidates the seismic damage mechanisms and failure patterns in loess tunnel portal sections. Results indicate that the tunnel portal section constitutes as a typical zone of energy concentration and structural vulnerability, specifically manifested in the crown at depths of 3.8d-5.2d (where d denotes the tunnel diameter), the portal area, and the invert within 1.5d-3d. These regions are characterized as high-intensity areas in Arias intensity cloud map and are prone to failures such as crown collapse, horizontal deformation, or invert heave. The damage evolution process exhibits distinct stage-specific characteristics. When the structure enters the irreversible plastic damage stage, spatial abrupt change and sharp increase in Arias intensity values occur, accompanied by a significant rise in the proportion of low-frequency energy (E1) in the wavelet packet energy spectrum, indicating that low-frequency energy dominates structural failure and that a significant seismic amplification effect exists in the portal section. Furthermore, complex interactions occur between the tunnel structure and the surrounding soil. The tunnel stiffness alters the dynamic response and stress field of adjacent soil, but this constraining effect gradually diminishes as soil damage accumulates, thereby exacerbating dynamic instability. The findings provide critical insights for the seismic design of tunnel portal sections, particularly in handling stiffness transition zones and formulating frequency-domain design criteria.