Traditionally, land subsidence resulting from groundwater over-pumping has often been described by the theory of consolidation. The mechanism of land subsidence due to the dehydration of clay minerals has not been well addressed. Therefore, this study develops a smectite dehydration model to describe the effect of the release of water from the smectite interlayer upon land subsidence. Using a thermodynamic solid-solution model and laboratory studies of clay-water systems, a complete description of the dehydration relationships among the swelling pressure, basal spacing and mw/mc in a Na-smectite water system at 25°C under variable pressure conditions was derived. Accordingly, the evaluation model of the ‘hydration state of smectite’, and the ‘solid-solution model of smectite dehydration’ were formulated rigorously. These two models were applied to quantify the effects of smectite dehydration on the accumulated land subsidence in the Yun Lin offshore industrial infrastructure complex and in the Tai-Shi area. The result reveals that smectite dehydration is of importance in assessing and predicting land subsidence in a shallow sedimentary basin.