Polymer　infiltration　is　an　efficient　way　to　strengthen　carbon　nanotube　fibers　in　the　process　of　spinning.　Given　the　carbon　nanotube/polymer　composite　fibers　obtained　from　this　approach　possess　higher　mechanical　properties　than　the　ones　of　their　components,　the　general　rule　of　mixtures　for　a　composite　material　is　not　applicable.　Accordingly,　a　novel　coarse-grained　molecular　dynamics　model　is　proposed　to　investigate　the　mechanical　behavior　and　microstructural　evolution　of　the　composite　fibers,　in　which　the　carbon　nanotubes　and　polymer　chains　are　represented　by　mesoscale　coarse-grained　beads.　Then　the　reinforcing　mechanism　of　the　composite　fibers　is　illustrated.　Dependence　of　the　mechanical　behavior　on　the　microstructure　of　the　composite　fiber　under　different　strain　rates　is　revealed　as　well.　Furthermore,　the　variations　of　mechanical　properties　of　the　composite　fibers　with　cycle　numbers　under　relatively　low　load　are　predicted.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.