Seawater　hydraulic　axial　piston　motor　is　an　important　and　elemental　component　in　underwater　tool　system.　The　torque　characteristics　for　a　swash-plate-type　seawater　hydraulic　axial　piston　motor　is　investigated,　and　an　integrated　torque　model　for　the　motor　with　symmetrical　pre-compression　angles　has　been　developed,　which　consists　of　a　torque　sub-model　and　a　dynamic　pressure　sub-model.　Numerical　simulations　have　been　carried　out　to　examine　the　effects　of　(a)　pre-compression　angle,　(b)　relief-groove　obliquity,　(c)　motor　speed,　(d)　piston　chamber　dead　volume,　(e)　friction　on　the　dynamic　pressure　and　the　output　torque　characteristics.　The　results　indicate　that　the　pre-compression　angle,　the　friction　coefficient,　and　the　clearance　between　cylinder　bore/piston　have　significant　impact　on　the　torque　characteristics.　The　test　verification　has　been　undertaken　with　a　five　piston　water　hydraulic　motor.　This　research　contributes　to　the　mechanism　of　output-torque　fluctuation　in　a　swash-plate-type　seawater　hydraulic　axial　piston　motor,　as　well　as　the　investigation　of　the　torque　transition　phenomenon　owing　to　the　pre-compression　angle.　The　research　has　laid　the　foundation　for　the　development　and　improvement　of　the　seawater　hydraulic　axial　piston　motor　in　underwater　tool　system.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.