It is no doubt that there is plenty of “water” (existential form: H₂O, H₂ and (HO)⁻) existing in the mantle which can either exist in the form of fluids and melts or exist in aqueous minerals, nominally anhydrous minerals (NAMs) and dense hydrous Mg-silicates (DHMS). In this paper, “fluid” mainly refers to water which includes elements and compounds that dissolved in or migrated with it. Fluids, mainly consisting of water, play important roles in major geologic processes such as subduction zone earthquakes, mantle partial melting, magmatism and submarine hydrothermal activities. The subduction zone is a key place where the hydrated oceanic lithospheric plate subducts into the earth’s depth. And it is also an important zone of crust-mantle interaction. In the subduction zone, fluids are carried into the deep earth by the subducting lithospheric plate, part of the fluids are released into the overlying mantle wedge by extrusion, frictional heating and metamorphism, thereby lowering the melting point of mantle materials and causing magmatism; on the one hand, the ascending magma heats seawater that penetrates through cracks or rifts; on the other hand, the heated seawater and post-magmatic fluids generated by magma cooling compose the material basis of modern submarine hydrothermal activities; submarine hydrothermal activities not only affect the material composition of ocean water and ecological environment by importing a large number of underground elements or substances into ocean water, but also lead to the formation of hydrothermal polymetallic ore deposits with important economic value. Therefore, the fluid is a medium in the plate subduction process and the various important geological processes caused by it, thus it is a tracer to study these important geological processes. Based on the analysis of the main geological processes of fluids in the oceanic lithosphere subduction, this paper discussed the roles of fluids in earthquake mechanism, magmatic processes, modern submarine hydrothermal activities and subduction zone fluid mineralization. Furthermore, it is suggested that the recent research work should focus on these four aspects: (1) Making the assessment of the fluid flux subducted into the deep earth more accurate to solve global geochemical or material circulation problems. (2) Describing physical and chemical behaviors of fluids in plate subduction comprehensively and accurately, establishing theoretical models of fluid geological processes in subduction zones. (3) Making full use of modern tests and analysis methods, and obtaining accurate data in terms of in-situ analysis of minerals, testing physical and chemical indexes of fluid inclusions in minerals, stable and radioactive isotope analysis, etc., so as to find out the state and process of substances deep underground which can not be reached by current sampling and observation methods. (4) Developing numerical simulation technique to establish theoretical models of geological processes of fluids in subduction zones.