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Red-layer mudstone is a typical expandable rock, that is, it easily expands and deforms in water. This study focused on the dynamic quantitative analysis of the mudstone hygroscopic process and development of strain caused by the hydration expansion of clay, by combining a self-designed simple self-imbibition device with the digital image correlation method. The mudstone samples were self-imbibed in deionized water and salt solutions of different concentrations under the same atmospheric conditions (25°C and 50% humidity). The effects of the salt type and salt concentration on the water absorption quality, expansion rate, crack initiation and expansion, matrix deformation, and water migration of mudstone over time were analyzed. The change in the water absorption capacity of mudstone with time was determined from the water absorption characteristic curve. The location and strain development of the local high-strain area during the water–rock interaction was captured from the full-field strain cloud image. The migration of water on the rock surface at different times was observed using camera photographs. Finally, water absorption was combined with strain, water migration, and crack propagation. The effect of saltwater on the hygroscopic expansion of mudstone was evaluated. [ABSTRACT FROM AUTHOR]