Landslides in loess slopes are mainly triggered by heterogeneity in hydrogeology. The southern Jingyang Plateau (China) was selected to characterize the geological structure and groundwater synergetic mechanisms that trigger landslides through groundwater–soil interactions. The study investigated hydrogeological and irrigation factors, the groundwater-perching effect of paleosols, the groundwater-conducting effect of fissures, and the damming effect of slide mass accumulation. A two-dimensional steady unsaturated numerical seepage model of a vertical section was established to simulate landslide evolution induced by irrigation and fissures. Long-term irrigation causes springs and weak zones to develop when the water table rises above the slope foot. Perched groundwater can lead superfluous water to migrate to the slope edge. The downward expansion of fissures and water migration can mutually cause extension of weak zones beneath fissures. A slip zone may appear when the weak zone connects to weaknesses at the slope foot, which forms a larger foot along the slope. Landslides occur frequently under the combined action of hydrostatic pressure from water-filled cracks, dynamic water pressure from a rising water table, and depressed sliding resistance caused by water enrichment in soil. After sliding, the slide mass will accumulate at the slope foot, blocking the original groundwater drainage point, so the water level may rise again, leading to subsequent landslides developing with new fissures and continued irrigation. Investigations in areas irrigated by groundwater wells showed that pumping can lower the water table in the soil and eliminate weak zones at the slope foot, effectively reducing the occurrence of landslides.