Abstract: The frequent occurrence of landslides and avalanches on bedrock islands is a major hazard threatening their ecological security and land resources. Mudstone within sedimentary rocks often forms a weak sliding surface due to its water-softening behavior. This study investigates the red sandy mudstone on the Sandaoliang slope of Beichangshan Island. We identified its mineral composition and content, examined the variation of water content in rock samples over time under humidification, and analyzed the swelling and shear mechanical characteristics at different moisture contents. The results show that the sandy mudstone exhibits significant 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＜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 negatively exponentially, the cohesion reduction rate first decreases and then increases, and the internal friction angle shows a relatively smaller decline. These mechanical degradations indicate that prolonged rainfall can severely compromise slope stability. This study provides a theoretical basis and essential data for subsequent 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.