The oceanic subduction zone is the locality where the magmatic activity, the high- to ultra- high- pressure metamorphism, the intermediate- depth earthquake, mass transfer between crust and mantle, the element recycling and the large- scale mineralization of copper and gold occur concentratedly on the earth. The H2O- rich fluid not only controls these geological processes but also deeply influences the global recycling of volatiles such as carbon, sulfur and etc. After the altered oceanic lithosphere and the overlying sediments are subducted into the intermediate to the deep depth to a subduction zone (15~300 km), the fluid is continuously released by the breakdown of hydrous minerals at different depths, accompanied by the prograde metamorphism from the prehnite- pumpellyite facies to ultra- high- pressure eclogite facies. The fluid released by the subducted oceanic crust is mainly aqueous in most subduction zones, except in an ultra- high geothermal environment. However, the characteristics of fluids obviously vary with the depth at which the fluid is generated. The high- pressure to ultra- high- pressure terranes exposed in global orogens have preserved robust evidence of the fluid activity, such as segregations, veins, hydro- breccias and etc. The primary fluid inclusion entrapped in omphacite, garnet, epidote and other minerals is a direct record of fluids. This fluid is a type of chloride- bearing dilute aqueous solution containing the components, such as CO2-3, SO2-4, HS- and etc. at the intermediate depth (＜65 km) in a subduction zone. Furthermore, the major element solute (Si, Al, Ca, Mg, Fe, Na) content of fluids has two to three times the total dissolved solids of seawater and the trace element concentration displays the enrichment of large ion lithophileelements (LILEs) and light elements (B and Li), and the depletion of high field strength elements (HFSEs). Moreover, the fluid will change to the volatile (CH4, C2H6, H2S and etc.)- bearing solution with a character resembling the supercritical fluid when the subduction depth is over 65 km. The content of major element solutes has remarkably increased and has also loaded considerable concentrations of HFSEs and transitional metallogenic elements besides LILEs. The depth between 65~100 km in a subduction zone is the interval where the aqueous fluid has been gradually transformed to the supercritical one. This fluid displays a “sub- supercritical” character. The redox property and the element dissolving capacity have transformed gradually, accompanied by the final consumption of lawsonites and amphiboles. The O, Sr, Nd and metal isotopic tracking studies of high- pressure metamorphic rocks and related veins indicate that the various fluid sources have included altered basic oceanic crusts, altered mantle peridotites and sediments and have also preserved the imprint of seafloor hydrothermal alterations. The channelized fluid is transferred and episodically transported along the networking fractures. The scale of fluid flow can reach up to a km- scale. The duration time of fluid activity varies from several months to several hundred years. Although considerable understanding of subduction- zone fluids has been achieved up to now, many controversies and scientific problems remain to be explored in the future.