Flexible　lithium　sulfur　batteries　with　high　energy　density　and　good　mechanical　flexibility　are　highly　desirable.　Here,　we　report　a　synergistic　interface　bonding　enhancement　strategy　to　construct　flexible　fiber-shaped　composite　cathodes,　in　which　polypyrrole@sulfur　(PPy@S)　nanospheres　are　homogeneously　implanted　into　the　built-in　cavity　of　self-assembled　reduced　graphene　oxide　fibers　(rGOFs)　by　a　facile　microfluidic　assembly　method.　In　this　architecture,　sulfur　nanospheres　and　lithium　polysulfides　are　synergistically　hosted　by　carbon　and　polymer　interface,　which　work　together　to　provide　enhanced　interface　chemical　bonding　to　endow　the　cathode　with　good　adsorption　ability,　fast　reaction　kinetics,　and　excellent　mechanical　flexibility.　Consequently,　the　PPy@S/rGOFs　cathode　shows　enhanced　electrochemical　performance　and　high-rate　capability.　COMSOL　Multiphysics　simulations　and　density　functional　theory　(DFT)　calculations　are　conducted　to　elucidate　the　enhanced　electrochemical　performance.　In　addition,　a　flexible　Li-S　pouch　cell　is　assembled　and　delivers　a　high　areal　capacity　of　5.8　mAh　cm(-2)　at　0.2　A　g(-1).　Our　work　offers　a　new　strategy　for　preparation　of　advanced　cathodes　for　flexible　batteries.