Dual-gradient　drilling　technology　is　being　increasingly　used　in　formations　with　narrow　pressure　margins.　For　dual-gradient　drilling　based　on　downhole　separation,　hollow　spheres　are　separated　into　the　annulus　at　the　separator　position,　resulting　in　variable　mass　flow　in　the　wellbore.　Thus,　existing　heat　transfer　models　are　no　longer　suitable　for　describing　wellbore　temperature　profiles　in　dual-gradient　drilling.　This　study　focused　on　developing　a　wellbore　heat　transfer　model　that　fully　considers　separated　hollow　spheres　entering　the　annulus,　complex　casing　programs,　and　heat　sources,　for　dual-gradient　drilling　based　on　downhole　separation.　The　model　was　solved　using　an　iterative　method.　Then,　the　accuracy　of　the　model　was　verified　using　temperature　data　measured　from　two　wells.　Finally,　the　difference　in　the　annular　temperature　distributions　between　dual-gradient　drilling　and　conventional　single-gradient　drilling　were　investigated,　as　were　the　wellbore　heat　transfer　characteristics　for　dual-gradient　drilling.　The　following　major　conclusions　were　drawn:　(1)　for　dual-gradient　drilling　based　on　downhole　separation,　at　the　separator　location,　the　annular　fluid　temperature　does　not　decrease,　but　rather　increase　in　the　flow　direction　because　of　the　inflow　of　hollow　spheres;　(2)　a　clear　inflection　point　exists　in　the　annular　fluid　temperature　curve　at　the　location　where　the　separator　would　be;　(3)　the　magnitude　of　the　mutation　of　the　temperature　curve　at　the　inflection　point　is　considerably　affected　by　the　heat　capacities　of　the　hollow　spheres　and　the　pure　drilling　fluid;　(4)　under　the　same　change　in　separation　efficiency,　distance　between　the　bit　and　separator,　flow　rate,　and　thermal　conductivity　of　formation,　the　variation　range　of　the　fluid　temperature　at　the　bottom　hole　is　greater　than　that　at　the　wellhead.　©　2020　Tongji　University　and　Tongji　University　Press