A　bistable　composite　tape-spring　(CTS)　structure　is　a　thin-walled　open　slit　tube　with　fibres　oriented　at　+/-　45　degrees,　which　is　stable　at　both　the　extended　and　fully　coiled　configurations.　Owning　to　its　positive　Gaussian　curvature　deformation　mechanics　and　high　stowage-to-pack　ratio,　it　has　been　successfully　applied　and　launched　to　International　Space　Station　and　microsatellites　to　construct　deployable　solar　sails.　Intelligent　driving　designs　of　the　CTS-based　deployable　structures　are　becoming　more　and　more　important　to　further　reduce　weight　and　complexities　for　space　applications.　Here,　we　presented　novel　findings　on　the　passive　thermal　driving　mechanics　of　the　bistable　CTS　structure.　This　is　achieved　by　exploring　the　thermal　energy-induced　microstructural　expansion　and　contraction,　which　would　change　the　structural　curvature,　and　thus　regulating　the　strain　energy　within　the　CTS.　An　analytical　model　on　the　strain　energy　evolution　under　thermal　effects　was　established　to　predict　the　minimum　stable　shape　transition　paths,　as　well　as　to　determine　the　critical　boundary　conditions　for　thermal　driving.　Both　experiments　and　finite　element　model　were　then　carried　out　to　reveal　underlying　mechanisms.　It　is　found　that　a　CTS　is　able　to　be　passively　deployed　under　thermal　energy,　there　is　a　minimum　energy　constraint　to　initiate　the　shape　morphing　process,　and　the　critical　boundaries　are　dependent　on　the　thermal　expansion　of　the　structural　material.　These　findings　provide　a　novel　low　cost,　simple　and　reversed　smart　morphing　design　principle　of　the　CTS　structure,　enriching　the　theoretical　analysis　and　deployable　control　of　the　bistable　composites　to　benefit　future　deep　space　explorations.