Coiled　tubing　(CT)　has　been　increasingly　used　in　offshore　oil　and　gas　engineering.　However,　a　number　of　issues　should　be　solved　to　guarantee　reliability　and　safety　of　the　application.　In　this　study,　a　mechanical　model　of　CT-in-riser　system　is　extracted　from　actual　operations.　Moreover,　micro　elements　of　the　CT　and　riser　are　respectively　analyzed　using　the　infinitesimal　method,　and　then　a　coupling　model　is　derived.　After　differential　equations　are　replaced　by　algebraic　equations,　the　Newton　iteration　method　is　used　to　solve　the　coupling　model.　To　validate　above　theory,　an　experimental　platform　based　on　the　similarity　laws　is　built.　There　are　two　types　of　experiments　to　be　investigated,　including　the　effects　of　fixed　and　floating　boundaries,　and　the　effects　of　three　annular　clearances.　The　results　show　that　the　force-out　has　a　similarly　increasing　trend　along　the　force-in　under　the　fixed　and　floating　boundaries,　and　the　larger　the　annular　clearances,　the　lower　the　transfer　efficiency　of　the　axial　force.　The　maximum　error　between　calculation　results　and　experimental　results　is　within　±5%,　which　validates　the　accuracy　of　the　coupling　model.　The　study　will　provide　a　theoretical　support　for　CT＇s　operations　in　offshore　engineering.　©　2021