The Subduction system is a natural laboratory to investigate the interaction between the Earth’s hydrosphere and lithosphere. Water carried in by down-going slabs significantly affects the tectono-dynamic processes in the subduction zones, for examples the mantle serpentinization of the subducting slab, formation of magmas and active volcanic arcs, and the seismogenic behaviors of the subduction zone. Along the circum-Pacific subduction zones, the regional seismic phenomena can be well explained by the water content estimated from active source seismic experiments, and the low-velocity anomalies of the upper mantle detected by passive source seismic surveys are consistent with the fault development on the subduction slabs. Both the multichannel seismic reflection exploration and numerical simulation reveal that normal faults exist widely on subduction slabs, which may penetrate the Moho and reach a depth as deep as 20 km at least below the seafloor. Those normal faults can provide channels for fluid to enter the crust and upper mantle, resulting in serpentinization up to 1.4 wt% or even higher of the upper mantle. As the temperature and pressure increase during the subduction process, fluids with different characters may be released from dehydration and interact with the mantle at different depths. The water-rock interaction exists extensively at subduction zones as revealed by studies of fluid inclusions and metasomatic minerals. This paper highlights the recent research progress on water content detection and water-rock interaction of subduction slabs and discusses implications for future researches on hydrosphere-lithosphere interaction.