Taking the highly weathered mudstone cutting slope of Longquanhu Expressway in Jianyang City as the research object, a slope scaling model based on the principle of similarity was used to conduct slope tests under different rainfall intensities and durations. By monitoring the entire process of slope rainfall, the influence of different rainfall conditions on slope stability was analyzed. The research results indicate that the large porosity and hydrophilicity of strongly weathered mudstone are the material basis for slope instability, and the rapid dissipation of slope matrix suction after rainfall is the essential reason for instability. In the early stage of rainfall, heavy rainfall causes a sharp increase in the volumetric moisture content and pore water pressure of the slope, and a sudden drop in matric suction, which is inversely proportional to the intensity of rainfall. Early heavy rainfall will accelerate slope fai-lure and shorten the time for slope failure, and the duration of rainfall directly determines whether slope failure will occur. The surface deformation of the slope is mainly caused by tension cracks at the top of the slope, which develop to the surface and internal depth of the uphill slope. The distribution of volumetric water content and pore water pressure at the top of the slope changes more than that of the uphill slope, leading to the first failure of the top of the slope and gradually developing to the uphill slope.