To　ensure　safe　drilling　with　narrow　pressure　margins　in　deepwater,　a　new　deepwater　dual-gradient　drilling　method　based　on　downhole　separation　was　designed.　A　laboratory　experiment　was　conducted　to　verify　the　effectiveness　of　downhole　separation　and　the　feasibility　of　realizing　dual-gradient　in　wellbore.　The　calculation　of　dynamic　wellbore　pressure　during　drilling　was　conducted.　Then,　an　optimization　model　for　drilling　parameters　was　established　for　this　drilling　method,　including　separator　position,　separation　efficiency,　injection　volume　fraction,　density　of　drilling　fluid,　wellhead　back　pressure　and　displacement.　The　optimization　of　drilling　parameters　under　different　control　parameters　and　different　narrow　safe　pressure　margins　is　analyzed　by　case　study.　The　optimization　results　indicate　that　the　wellbore　pressure　profile　can　be　optimized　to　adapt　to　the　narrow　pressure　margins　and　achieve　greater　drilling　depth.　By　using　the　optimization　model,　a　smaller　bottom-hole　pressure　difference　can　be　obtained,　which　can　increase　the　rate　of　penetration　(ROP)　and　protect　reservoirs.　The　dynamic　wellbore　pressure　has　been　kept　within　safe　pressure　margins　during　optimization　process,　effectively　avoiding　the　complicated　underground　situations　caused　by　improper　wellbore　pressure.