Abstract　Seawater　hydraulic　axial　piston　pump　(SHAPP)　is　a　critical　power　component　in　underwater　operating　systems.　A　parameterized　elasto-hydrodynamic　(EHD)　lubrication　model　of　the　piston/cylinder　tribopair　in　the　SHAPP　is　originally　established,　which　considers　the　elasto-hydrodynamic　behavior,　viscosity　temperature　effect　and　deep-sea　environmental　pressure.　The　deformation　of　piston　bush,　bearing　mechanism　and　energy　loss　characteristics　of　the　water　film　under　different　operating　conditions　are　discussed.　The　results　show　that　the　deformation　of　piston　bush　made　of　polyetheretherketone　(PEEK)　is　in　micron-scale　under　water　film　and　deep-sea　pressure,　which　could　increase　the　leakage　and　viscous　friction　power　loss　of　piston/cylinder　tribopair.　In　deep-sea　environment,　the　leakage　and　viscous　friction　power　loss　calculated　by　EHD　are　always　greater　than　those　calculated　by　hydrodynamic　(HD),　and　increase　with　the　increase　of　working　pressure,　shaft　speed,　gap　size,　sea　depth　and　piston　bush　thickness.　Besides,　the　increase　of　sea　surface　temperature　could　increase　the　leakage　and　decrease　the　viscous　friction　power　loss　of　the　piston/cylinder　tribopair,　respectively.　Finally,　design　instructions　to　optimize　the　piston/cylinder　tribopair　are　presented,　therefore　the　proposed　methodology　can　be　used　as　a　designing　tool　for　SHAPP.　©　2020　Elsevier　Ltd