Abstract: Water absorption is a primary factor contributing to the swelling and disintegration of mudstone. This study aims to investigate the water absorption characteristics and mechanisms of red-bed mudstone from the central Sichuan Basin in both its original buried state and excavated state. Capillary water absorption tests were conducted on Penglaizhen Formation mudstone under both unconfined and confined conditions. X-ray diffraction and scanning electron microscopy were employed to comprehensively evaluate the water absorption behavior, microscopic mechanisms, and influencing factors. The results indicate that lateral confinement significantly reduces both the saturated water absorption rate and the maximum water absorption rate of the rock specimens. Under unconfined conditions, the capillary water absorption curves of the mudstone predominantly exhibited an S-shape, with the saturated water absorption rate showing a positive correlation with clay mineral content (R²=0.98) and a negative correlation with detrital particle content (R²=0.96). Under confined conditions, the capillary absorption curves displayed an upward convex shape, and the saturated water absorption rate was positively correlated with initial porosity (R²=0.83). Smaller pore spaces were associated with a stronger capacity for capillary water conduction, while larger pores (> 1 μm) played a dominant role in increasing the water absorption rate. At the microscopic level, the capillary water absorption process of mudstone can be divided into three stages, with lateral constraints primarily limiting water absorption in the third stage. These findings provide a scientific basis and theoretical support for understanding the disintegration and swelling mechanisms of red-bed mudstone.