Subducting sedimentary layer typically contains water and hydrated clay minerals. The stability of clay minerals under such hydrous subduction environment would therefore constraint the lithology and physical properties of the subducting slab interface. Here we show that pyrophyllite (Al₂Si₄O₁₀(OH)₂), one of the representative clay minerals in the alumina-silica-water (Al₂O₃-SiO₂-H₂O, ASH) system, breakdowns to contain further hydrated minerals, gibbsite (Al(OH)₃) and diaspore (AlO(OH)), when subducts along a water-saturated cold subduction geotherm. Such a hydration breakdown occurs at a depth of ~135 km to uptake water by ~1.8 wt%. Subsequently, dehydration breakdown occurs at ~185 km depth to release back the same amount of water, after which the net crystalline water content is preserved down to ~660 km depth, delivering a net amount of ~5.0 wt% H₂O in a phase assemblage containing δ-AlOOH and phase Egg (AlSiO₃(OH)). Our results thus demonstrate the importance of subducting clays to account the delivery of ~22% of water down to the lower mantle.