Due　to　its　unique　advantages,　coiled　tubing　is　now　widely　used　in　marine　oil　and　gas　exploitation　operations,　while　the　mechanical　behavior　of　coiled　tubing　working　within　a　marine　riser　is　still　unknown.　The　mechanical　model　of　this　special　pipe-in-pipe　system　was　built　by　analyzing　the　force　of　coiled　tubing　and　marine　riser,　using　quasi-static　method.　And　the　model　was　verified　by　experimental　study.　Based　on　the　established　model,　the　effects　of　different　coiled　tubing　dimensions　and　platform　transversal　on　the　axial　force　of　coiled　tubing　within　a　marine　riser　were　analyzed.　It　was　found　that　increasing　the　size　of　the　coiled　tubing　can　improve　the　axial　load　of　the　coiled　tubing　without　affecting　the　force　of　the　riser.　While　changing　the　platform　transversal　would　have　less　impact　on　the　coiled　tubing,　and　a　greater　impact　on　the　riser＇s　force.　The　mechanical　model　built　can　provide　theoretical　guidance　on　coiled　tubing　marine　operations.　©　2020　IOP　Publishing　Ltd.　All　rights　reserved.