Abstract: Sandstone joints are among the key geological factors controlling the development of geohazards in red-bed regions. A thorough understanding of joint characteristics and their formation mechanisms is therefore essential for addressing engineering geological problems associated with the red-bed strata of the Sichuan Basin. This study combined field investigations and discrete element modeling to examine the joint spacing-layer thickness relationship and its formation mechanisms in Sichuan Basin red-bed sandstone. Field survey results indicate that the joint spacing-to-thickness ratio of red-bed sandstone in the Sichuan Basin ranges from 0.640 to 1.470, while numerical simulations yield average ratios (1.188 and 1.090) consistent with field data. Creep simulations based on the Bonded Particle Model with Stress Corrosion reveal that mudstone rheology induces interlayer shear stress, generating tensile stress concentration zones in the mid-sections of sandstone layers and promoting vertical joint formation. Statistical analysis indicates that joint spacing follows a normal distribution and presents a strong linear correlation with layer thickness, approximating a 1:1 ratio. The stress gradient distribution in sandstone layers conforms to the shear lag theory model. The shear lag effect, resulting from the creep differential between sandstone and mudstone, is the primary mechanism governing the development of vertical joints in sandstone. These findings provide a theoretical foundation for understanding geohazard evolution and mitigation strategies in red-bed regions of the Sichuan Basin.