In　the　water　jet　propulsion　system　with　a　positive　displacement　(PD)　pump,　the　nozzle,　which　converts　pressure　energy　into　kinetic　energy,　is　one　of　the　key　parts　exerting　great　influence　on　the　reactive　thrust　and　the　efficiency　of　the　system　due　to　its　high　working　pressure　and　easily　occurring　cavitation　characteristics.　Based　on　the　previous　studies　of　the　energy　loss　and　the　pressure　distribution　of　different　nozzles,　a　model　of　water　jet　reactive　thrust,　which　fully　takes　the　energy　loss　and　the　nozzle　parameters　into　consideration,　is　developed　to　optimize　the　nozzle　design.　Experiments　and　simulations　are　carried　out　to　investigate　the　reactive　thrust　and　the　conversion　efficiency　of　cylindrical　nozzles,　conical　nozzles　and　optimized　nozzles.　The　results　show　that　the　optimized　nozzles　have　the　largest　reactive　thrust　and　the　highest　energy　conversion　efficiency　under　the　same　inlet　conditions.　The　related　methods　and　conclusions　are　extended　to　the　study　of　other　applications　of　the　water　jet,　such　as　water　jet　cutting,　water　mist　fire　suppression,　water　injection　molding.