Continuous　macroscopic　carbon　nanotube　(CNT)　fibers　are　made　of　meso-scale　CNT　bundles,　where　most　CNTs　are　well　aligned　along　the　fiber　axis　direction.　Super-stretchable　helical　composite　fibers　made　from　the　pure　CNT　fibers　are　promising　candidates　for　applications　in　stretchable　and　wearable　devices.　This　paper　presents　multi-scale　molecular　dynamic　simulations　of　CNT　fibers　and　super-stretchable　helical　composite　fibers.　Simulations　of　monotonic　and　cyclic　tensile　tests　were　conducted　to　study　their　plastic　deformation　and　micro-structural　evolution.　It　is　found　that　the　elongation　of　the　helical　structures　can　reach　100%~300%,　depending　on　the　pitch　of　the　helix　and　the　tensile　strain　rate.　The　specific　strength　of　the　helical　structures　greatly　decreases　for　a　decreasing　helix　pitch.　The　deformation　mechanisms　are　divided　into　four　types　according　to　different　predominate　factors,　for　different　strain　rates.　Besides,　the　reinforcing　mechanism　of　the　composite　fibers　is　illustrated.　Dependence　of　the　mechanical　behaviour　on　the　microstructure　of　the　composite　fibers　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.　©　2017　International　Committee　on　Composite　Materials.　All　rights　reserved.