Gas　pollution　in　marine　lubricating　oil　systems　is　harmful　to　the　normal　operation　of　a　ship,　and　is　one　of　the　main　reasons　for　the　decline　of　the　performance　of　lubricating　oil.　In　this　research,　a　classic　75　mm　hydrocyclone　was　selected　as　the　oil-gas　separation　device.　A　hydrocyclone　is　a　device　that　uses　the　density　difference　of　the　two-phase　flow　to　separate　the　dispersed　phase　in　the　centrifugal　force　field.　Compared　with　ordinary　active　oil-gas　separators,　hydrocyclones　do　not　require　additional　power　devices.　After　establishing　the　physical　model　of　the　hydrocyclone,　the　distribution　characteristics　of　the　flow　field　and　oil-gas　two-phase　flow　separation　performance　of　the　hydrocyclone　were　studied　using　computational　fluid　dynamics　(CFD)　technology.　The　influence　of　vortex　finder　diameter,　vortex　finder　length,　spigot　diameter,　cylindrical-part　length,　and　cone　angle　on　the　oil-gas　separation　performance　of　the　hydrocyclone　were　investigated.　It　was　found　that　the　vortex　finder　diameter　and　the　spigot　diameter　have　a　significant　influence　on　the　oil-gas　separation　performance,　whereas　the　vortex　finder　length,　the　cylindrical-part　length,　and　the　cone　angle　have　little　influence　on　its　performance.　Increasing　the　vortex　finder　diameter　and　reducing　the　spigot　diameter　can　improve　the　gas　separation　efficiency.　However,　the　liquid　outflow　from　the　vortex　finder　increases,　which　causes　the　liquid　loss　rate　to　increase.　The　presented　research　could　lay　a　foundation　for　the　optimal　design　of　a　hydrocyclone　used　for　oil-gas　separation　of　a　marine　lubricating　oil　system.