Abstract: Obtaining the groundwater seepage field is an important prerequisite for revealing landslide mechanisms, stability evaluation, and prevention design. For large landslides located within fault fracture zones, the rock mass exhibits extreme permeability heterogeneity, leading to highly complex groundwater seepage fields and rainfall infiltration patterns that are difficult to clarify using conventional investigation methods. Taking the Yahuokou large landslide in the Pingding-Huama fault zone in Zhouqu County, Gansu Province, as the study object, this research employed surface nuclear magnetic resonance, high-density electrical resistivity tomography, and self-potential method to conduct refined detection of the landslide's groundwater seepage field. Based on the geological model of the Yahuokou landslide, its rainfall infiltration pattern and the role of rainfall in landslide instability was analyzed. The results indicate: (1) The groundwater seepage field of the landslide is divided by two sliding zones into two relatively independent units: the unit above the first sliding zone and the unit between the first and second sliding zones. Since the detection was conducted just after the rainy season, both slope units were nearly saturated. (2) The slope body above the first sliding zone was saturated by direct rainfall infiltration through the slope surface. Due to the sliding zone acting as a relatively impermeable layer, it is inferred that rainfall first infiltrated vertically through preferential pathways along the fault fracture zones at the sides and rear of the landslide, followed by lateral recharge, gradually saturating the slope body between the first and second sliding zones. (3) The dynamic water pressure of groundwater and the buoyant force exerted on the first sliding zone may have been critical factors contributing to the occurrence of the Yahuokou landslide in 2009. This research provides valuable insights into the formation and evolution mechanisms, stability evaluation, and prevention design of landslides in fault fracture zones.