This　paper　aims　to　investigate　the　wellbore　thermal　behavior　in　multi-gradient　drilling　(MGD).　Based　on　the　principle　of　energy　conservation,　a　synthetic　transient　heat　transfer　model　was　established,　in　which　the　mass　and　heat　transfer　of　lightweight　glass　microspheres　(GMSs)　were　considered　for　the　wellbore　fluid　region,　and　the　dynamic　variation　in　the　moving　boundary　was　considered　for　the　drill　bit　and　separator　region.　The　synthetic　model　was　solved　using　the　combination　of　finite　volume　method　and　dynamic　laying　method.　The　accuracy　and　capability　of　the　model　were　verified　using　field　measurement　data　and　previous　typical　models.　The　calculation　results　suggested　that　the　annular　temperature　near　the　separator　dropped　suddenly　due　to　the　mass　and　heat　transfer　of　GMSs.　Additionally,　the　dynamic　drilling　of　the　drill　bit　would　increase　the　bottomhole　temperature　continuously.　Furthermore,　a　sensitivity　analysis　indicated　that　when　the　GMS　concentration　increased　from　5%　to　45%,　the　annulus　temperature　at　the　separator　reduced　by　9.15%;　when　the　distance　between　the　drill　bit　and　separator　increased　from　1100　m　to　3000　m,　the　bottomhole　temperature　increases　by　3.99%;　and　the　effect　of　the　separator　number　on　annulus　temperature　combined　the　dual　role　of　separator　position　and　GMS　concentration.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.