Abstract: Seawater hydraulics is successfully used in deep-sea equipment because of its environmental friendliness, seawater pressure automatic compensation, low operating cost, easy disposal of working media, simpleness of system composition and cleanliness. The advantages of seawater hydraulics and research profile are introduced. The key technical challenges and countermeasure of seawater hydraulics used in deep sea equipment are analyzed. Meanwhile, the effect of deep sea environment on seawater hydraulic components, such as the effect of seawater pressure and temperature on working media, particle pollution of seawater and the effect of seawater pressure on friction pair are analyzed, the solutions based on new principle, new materials, new structures and new procedures are further proposed. Three applications of seawater hydraulics in deep-sea equipment are presented. One of them is the seawater hydraulic variable ballast system, which is a promising substitute for oil hydraulic buoyancy regulating system and variable ballast system based on pneumatic because of many advantages, such as simple mechanism and high-reliability. Furthermore, seawater hydraulic variable ballast system is a general choice of deep sea submersible vehicles. As new promising driven modes for underwater tools, seawater hydraulic manipulator and seawater hydraulic ray are in the presentation of the applications as well. Seawater hydraulic underwater tool is also a typical application. Compared with oil hydraulic underwater tools, it has simpler system composition, lower pressure loss, more convenient maintenance and higher efficiency.

Key words: seawater hydraulics;deep-sea equipment;variable ballast system;underwater tool;manipulator