In　order　to　improve　the　separation　efficiency　of　downhole　separator　for　multi-gradient　drilling　using　hollow　glass　spheres(HGS)　injection　method,　a　filter　separator　that　can　achieve　high-efficiency　separation　was　designed　and　investigated.　A　geometric　model　was　established　based　on　the　filter　structure　of　the　filter　separator,　and　the　correlation　between　the　diameter　of　the　separation　port　and　the　minimum　size　of　the　channel　was　further　derived.　For　the　internal　flow　inside　the　filter　structure,　numerical　simulation　was　conducted　to　study　the　jet　velocity　and　negative　pressure　variation　with　the　injection　velocity　at　the　separation　port.　The　numerical　simulation　coupled　with　the　porous　media　model　and　the　multiphase　flow　model　was　combined　with　the　laboratory　experiments　to　study　the　separation　efficiency　of　the　HGS　under　different　conditions.　A　model　to　calculate　the　pressure　fluctuation　when　the　mixed　fluid　entering　annulus　from　the　separator　port　was　also　derived　based　on　the　drilling　data.　The　results　show　that　the　separation　efficiency　of　the　filter　separator　increases　with　the　improvement　of　the　pump　flow　rate　and　the　HGS　volume　fraction,　in　which　a　high　value　of　98%　can　be　achieved.　The　split　ratio　at　the　separation　port　is　5.7%,　which　has　a　limited　impact　on　the　pump　flow　rate.　©　2021,　Editorial　Office　of　Journal　of　China　University　of　Petroleum(Edition　of　Natural　Science).　All　right　reserved.