Abstract: Designing and constructing highway bridges in high-intensity mountainous areas present significant challenges. The stability of high and steep bank slopes for large span bridges coupled with various unfavorable conditions under strong earthquakes is particularly complex, which is prone to formation of bank slope instability disasters such as sliding and debris flow. Investigations into earthquake damage reveal that irregular terrain has a significant amplification effect on earthquake dynamics, which has an adverse impact on the stability of slopes and the safety of bridges. Assessing the seismic dynamic amplification effect of complex terrain is of important engineering value. This study examines the bank slope of a 1200m-long suspension bridge located in the high-intensity, deep canyon region of the Liangshan Yi Autonomous Prefecture, Sichuan Province. We conduct an in-depth analysis and research on the seismic hazard probability and instability failure mode mechanisms of the bedrock surface under strong seismic forces. A three-dimensional slope structure model with unloading cracks was developed. The peak seismic acceleration of each characteristic point on the bank slope under different instability failure modes was obtained using dynamic time-history analysis method and modified amplification coefficient was derived based on these findings. Improvements were made to the static calculation method for slope seismic stability using this modified coefficient. The improved method was used to evaluate the stability of the construction site. The results indicate that the slope's peak seismic horizontal acceleration and amplification coefficient are highest at the surface and decrease with increasing slope depth, with the rate of decrease slowing and stabilizing. The rate of slope change significantly impacts this response. The seismic response is exceptionally strong in areas with high slope change rates and prominent landforms. Widely distributed fragmented rock and soil cover layers, shallow surfaces with varying slope rates, and surface weathered fragmented rock masses within weathering unloading zones are prone to deformation under seismic action, and protection should be strengthened. The calculation results of seismic conditions without considering the correction of amplification factors are unsafe, with safety factor results decreasing by 2% to 6%. A complete set of geological hazard risk assessment methods, and slope stability calculation methods, and protective measures suitable for considering the two-level seismic resistance of bridge structures are proposed based on this for the bank slopes of ultra large span bridges in high intensity mountainous areas, providing a reference for the research and design of related engineering projects in high-intensity mountainous areas.