Variable　gradient　drilling　in　deep　water　is　a　new　drilling　method　of　using　separators　to　separate　hollow　glass　spheres　(Referred　to　as　HGS,　it　is　the　glass　microbeads　with　low　density　less　than　1.0　kg/m3,　and　the　worse　thermal　conductivity)　from　the　drill　string　directly　into　the　annulus,　which　can　form　multiple　gradients　of　drilling　fluid　density　in　annulus.　This　method　can　better　deal　with　the　problem　of　narrow　pressure　window　in　deep　water　drilling.　At　present,　there　are　few　studies　on　the　wellbore　temperature　and　pressure　distribution　of　this　technology,　and　the　separation　efficiency　of　the　existing　separator　is　too　low　to　achieve　variable　gradient.　The　purpose　of　this　paper　is　to　develop　a　new　separator　to　improve　the　separation　efficiency,　and　to　study　the　coupled　wellbore　temperature　and　pressure　field　of　variable　gradient　drilling　based　on　dynamic　mass　flow.　Firstly,　indoor　simulated　experiments　were　used　to　study　the　separation　efficiency　of　the　newly　developed　filter　separator,　and　based　on　the　separation　efficiency,　wellbore　flow　theory　and　the　first　law　of　thermodynamics,　the　dynamic　mass　flow　at　the　location　of　the　separator　were　fully　considered.　Considering　the　influence　of　wellbore　temperature　and　pressure　on　thermophysical　parameters,　a　coupled　model　of　wellbore　temperature　and　pressure　was　established　based　on　dynamic　mass　flow.　Then　the　model　was　discretized　and　solved　by　finite　difference　method　and　cyclic　iterative　algorithm.　Further,　this　model　was　verified　based　on　the　existing　models　and　drilling　site　data.　Finally,　the　effects　of　key　parameters　in　variable　gradient　drilling　on　the　annulus　temperature,　pressure　and　fluid　density　were　studied.　The　results　show　that　the　filter　separator　has　high　feasibility,　and　the　separation　efficiency　can　reach　95%–98.5%.　Compared　with　conventional　drilling,　distribution　curve　of　the　temperature,　pressure　and　the　mixture　density　in　annulus　of　the　filter　separator　section　has　obvious　inflection　points　in　variable　gradient　drilling,　and　the　number　of　inflection　points　is　equal　to　the　number　of　separators.　What＇s　more,　the　number　and　location　of　separators,　the　HGS　volume　fraction,　pump　flow　rate,　and　the　HGS　density　have　significant　effects　on　the　annulus　temperature　and　pressure.　This　research　can　significantly　improve　the　feasibility　of　variable　gradient　drilling　in　deep　water　and　provide　a　theoretical　reference　for　further　research　in　this　direction.　©　2021　Elsevier　B.V.