Large-scale landslide disasters are frequently triggered by heavy rainfall, posing significant threats to human life and property. The study of landslide formation mechanisms and rainwater infiltration patterns under heavy rainfall conditions is considered crucial for landslide monitoring, early warning, and prevention. The Youjiahe landslide was used as a case study to elucidate the deformation mechanism of the secondary sliding body. Years of macro-inspection data and fully automated monitoring data were analyzed. The fissure infiltration process induced by rainfall was elaborated. Geostudio software was employed for simulation analysis. The impact of varying rainfall intensities on the evolution characteristics of the secondary sliding body was further examined. The results show as follows. Heavy rainfall is identified as the primary factor triggering step-like deformations in secondary sliding bodies. Significant rainfall events are observed prior to each of four step-like deformation occurrences. Deformation responses generally exhibit a 1 ~ 2 day temporal lag relative to rainfall events. The Youjiahe landslide is characterized as a shallow landslide with gently sloping flat layers. Rainfall accumulation on slope surfaces is facilitated during runoff and infiltration processes. A unique "pore + fissure" composite infiltration mode is formed by this slope structure. The rainfall recharge process is jointly regulated by slope structure, rainfall intensity, and retaining wall presence. Landslide stability is consequently influenced through these regulatory mechanisms. Rainfall infiltration is initially observed in flat slope areas through numerical simulations. Rainwater discharge at the slope toe is impeded by retaining walls. Slope body displacement is simultaneously restricted by these retaining structures. The stability of secondary sliding body is progressively diminished with increasing rainfall intensity. The research findings are provided as references for landslide monitoring and early warning systems and preventive measures.