Abstract: A collapse accident occurred on a high-fill subgrade on slope of an expressway on a slope in the mountainous area of northern Guangdong under heavy rainfall. Based on the working conditions at the accident site, this paper investigates the instability mechanism of the high-fill subgrade on a slope under heavy rainfall. The GMS three-dimensional seepage model is adopted, with natural ridges around the site set as watershed of the model. A three-dimensional seepage field model is established according to geomorphological and stratigraphic conditions. The rainfall process within the computational domain is imported, and by referencing the time histories of groundwater levels at in-situ monitoring points, inversion analysis is carried out to reproduce the spatial distribution and variation characteristics of groundwater prior to the accident. Water accumulates in filled areas and low-lying zones adjacent to the sloped high-fill subgrade in valley sections during heavy rainfall, generating high hydraulic head pressure. Superimposed on the seepage in deep rock and soil mass, this combined effect triggers the instability of the sloped high-fill subgrade. Surface infiltration during heavy rainfall constitutes the dominant controlling factor governing the characteristics of the groundwater seepage field. Considering the construction cost and construction difficulty of protective measures, the scheme combining impermeable covering layer and drainage ditch exhibits the best cost-effectiveness for hydraulic protection within the study area.