Water　hydraulic　directional　valve　is　a　key　control　element　in　seawater　fluid　power　system.　Because　of　the　properties　of　high　bulk　modulus　and　low　viscosity　of　seawater,　the　leakage　and　dynamic　characteristics　of　directional　valve　would　be　different　from　those　of　the　traditional　one.　In　this　study,　a　new　working　principle　of　seawater　hydraulic　4-3　cartridge　directional　valve　(abbr.　SHCDV)　is　introduced,　and　the　mathematical　model　and　simulation　model　of　the　SHCDV　are　developed　in　this　paper.　The　pressure　and　flow　characteristics　are　investigated　through　simulation　and　experiment.　The　optimal　structure　parameters　and　dynamic　characteristic　curves　of　SHCDV　are　achieved　through　simulation.　The　simulation　results　show　that　the　control　chamber　area,　the　damping　hole　diameter　of　the　cartridge　valve,　and　the　flow　rate　of　the　pilot　valve　have　important　influences　on　the　dynamic　characteristics　of　SHCDV.　Meanwhile,　the　static　and　dynamic　characteristics　of　SHCDV　are　measured　using　a　water　hydraulic-component-performance　test　stand.　The　results　show　that　its　rated　pressure　is　14　MPa,　the　flow　rate　is　120　L/min,　the　dynamic　opening　response　time　is　109　ms,　and　the　closing　time　is　128　ms.　The　experimental　results　are　basically　consistent　with　the　simulation　results,　and　verify　the　accuracy　of　the　simulation　model.　This　research　will　lay　the　foundation　for　the　further　development　of　a　high-pressure,　high-flow,　and　fast-response　SHCDV,　and　it　can　be　widely　used　in　oceanic　exploration,　marine　development,　desalination,　and　other　fields.