Materials　with　intrinsically　low　thermal　conductivity　are　of　fundamental　interests.　Here　we　report　a　new　sort　of　simple　one-dimensional　(1D)　crystal　structured　bismuth　selenohalides　(BiSeX,　X　=　Br,　I)　with　extremely　low　thermal　conductivity　of　∼0.27　W　m−1　K−1　at　573　K.　The　mechanism　of　the　extremely　low　thermal　conductivity　in　1D　BiSeX　is　elucidated　systematically　using　the　first-principles　calculations,　neutron　powder-diffraction　measurements　and　temperature　tunable　aberration-corrected　scanning　transmission　electron　microscopy　(STEM).　Results　reveal　that　the　1D　structure　of　BiSeX　possesses　unique　soft　bonding　character,　low　phonon　velocity,　strong　anharmonicity　of　both　acoustic　and　optical　phonon　modes,　and　large　off-center　displacement　of　Bi　and　halogen　atoms.　Cooperatively,　all　these　features　contribute　to　the　minimal　phonon　transport.　These　findings　provide　a　novel　selection　rule　to　search　low　thermal　conductivity　materials　with　potential　applications　in　thermoelectrics　and　thermal　barrier　coatings.　©　2020,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.