Conventional　twistable　structures　use　discrete　parts　articulated　around　a　number　of　linkages.　These　allow　only　a　limited　degree　of　twisting　angle,　are　low　in　storage　ratio,　heavy　and　complex　in　morphing　mechanisms.　Doublehelix　structures　are　commonly　applied　to　induce　twistable　shape-changing　capability　for　deployable　structures,　these　being　capable　of　large　axial　deformations　where　prestressed　thin-shell　composite　flanges　or　strips　are　employed;　however,　their　structural　stabilities　are　susceptible　to　thermal　effects,　and　suffer　from　non-zero　Gaussian　curvature　deformation　induced　by　prestressing　of　the　precured　flat　strips.　Here,　we　propose　a　novel　bistable　helical　structure,　where　zero　Gaussian　curvature　deformation　applies,　and　shows　more　stable　and　reliable　morphing　mechanics　for　a　twistable　structure　to　be　engineered.　This　is　achieved　by　exploiting　bistable　composite　tape-spring　(CTS)　structures,　where　two　CTS　samples　are　pin-joined　through　spokes　to　formulate　a　helical　structure.　It　is　capable　of　large　axial　morphing,　and　stable　in　both　the　fully　extended　and　twisted　configurations,　with　adjustable　storage　ratio.　A　theoretical　model　was　established　to　predict　its　bistability　and　a　bespoke　axial　displacement　rig　was　developed　to　investigate　its　non-linear　morphing　mechanisms　in　order　to　reveal　the　underlying　fundamentals.　These　will　facilitate　torsional　structural　design　for　aerospace　deployable　structures.