Water in the mantle has an important influence on not only the nature of geophysical fields, but also geochemical dynamics, mantle transverse heterogeneity, mantle convection, low velocity layer, subduction zone, phase equilibrium, the behavior of melting, deep geological calamity (earthquake and volcanic eruption) and mineralization. So researchers had paid attention to study on water in the mantle for a long time. In order to discuss the role of water in the mantle, it is necessary to get a clear understanding of storage sites of water. Early studies show that there are three main storage sites of water in the mantle: hydroxyl (OH) in minerals, as free fluid, or in melts. This paper summarizes advance in study on storage sites of water in the mantle and puts forward some proposals of future study directions.