Abstract: The red beds in the southwestern mountainous regions of China exhibit a distinctive interbedded sandstone-mudstone structure with high water sensitivity. Tunnels constructed on dip slopes within these formations are particularly susceptible to catastrophic failure under sustained rainfall, especially during the post-construction phase. To investigate the failure modes of red-bed dip slopes with interbedded sandstone and mudstone and the deformation characteristics of tunnels under rainfall, this study develops physical model tests of three tunnel alignment schemes based on similarity theory. Variations in earth pressure, pore water pressure, and tunnel strain data are analyzed, combined with slope deformation and failure characteristics. This reveals the deformation mechanisms, slope instability patterns, and failure evolution under different tunnel alignment schemes. Results indicate that under rainfall, dip slopes in red-bed areas are prone to retrogressive progressive sliding, evolving through four stages: rainwater infiltration, fracture propagation, localized instability, and progressive sliding. Tunnel alignment significantly affects slope damage and structural deformation. Tunneling beneath soft red-bed strata triggers large-scale sliding failure with maximum slope damage but minimal tunnel deformation; tunneling above soft strata results in integral slope instability with a better-preserved sliding mass and shorter sliding distance, accompanied by moderate tunnel deformation; in contrast, tunneling directly through the soft layer results in only localized toe failure of the slope but causes the most significant deformation within the tunnel structure.