Abstract: The frequent occurrence of landslides and avalanches on bedrock islands poses a significant hazard to their ecological stability and land resources. Mudstone within sedimentary rocks often forms a weak sliding surface due to its susceptibility to water-softening. This study investigates the red sandy mudstone on the Sandaoliang slope of Beichangshan Island. We identified its mineral composition and content, examined the variation in water content of rock samples over time during humidification, and analyzed the swelling and shear mechanical properties at different moisture levels. The results indicate that the sandy mudstone exhibits considerable expansibility, with a volumetric expansion rate of 2.9% at saturation. During natural water absorption, the moisture content evolution can be divided into three stages: a uniform growth stage (0≤t≤30 h), a slow growth stage (30 h＜t≤130 h), and a stabilization stage (t≥130 h). The saturated moisture content reaches 16%-18%. As the moisture content increases, the uniaxial compressive strength decreases exponentially, the cohesion reduction rate initially decreases and then increases, and the internal friction angle exhibits a relatively minor decline. These mechanical degradations indicate that prolonged rainfall can significantly compromise slope stability. This study provides a theoretical basis and essential data for further research into the rainfall threshold that triggers slope instability on Beichangshan Island, thereby offering valuable insights and a foundation for the ecological restoration of bedrock island slopes.