Abstract: The stability of high fill slopes is affected by the coupling effect of multiple factors, and the identification of key control factors is of great significance for engineering design and safety evaluation. Taking the high fill slope of a substation in the western mountainous area as the research object, based on the positive and negative correlation differences of influencing factors, sensitivity analysis of factors affecting slope stability is carried out, and numerical simulation is combined to study the deformation characteristics under extreme working conditions. The results indicate that cohesion, internal friction angle, and unloading platform width are the dominant factors controlling the stability of high fill slopes, and their correlation is significantly higher than that of slope height, fill density, slope gradient, and seismic peak acceleration; Under the action of heavy rainfall, the matric suction in the upper part of the slope significantly decreases, and shallow deformation first occurs and gradually develops towards the deeper part; Under the action of an earthquake, there is a significant accumulation of plastic displacement in the upper part of the slope, with horizontal deformation dominating and an elevation amplification effect. The research results reveal the main controlling factors of the stability of high fill slopes and their deformation response laws under rainfall and earthquake conditions, which can provide a basis for the design, construction, and reinforcement of high fill slopes.