Te-free　Bi2S3-based　thermoelectric　materials　show　great　potential　for　eco-friendly　and　industrial　scale-up　applications　because　of　their　high-abundance,　low-cost,　low-toxicity,　and　low-thermal-conductivity　features.　However,　their　low　figure　of　merit,　ZT　limits　their　further　applications.　In　this　work,　we　report　a　high　ZT　of　similar　to　0.8　at　similar　to　760　K　in　n-type　polycrystalline　Bi2S3　by　a　combination　of　hierarchical　structure　manipulation　and　carrier　density　optimization.　A　step-by-step　fabrication　by　using　mechanical　alloying,　high-pressure　and　high-temperature　treatment,　spark　plasma　sintering,　and　annealing　leads　to　unique　micro/nanostructures　in　polycrystalline　Bi2S3　including　refined　grains,　high-density　Bi-rich　nanoprecipitates,　significant　lattice　distortions,　and　nanopores　that　confirmed　by　comprehensive　characterizations,　which　contribute　to　significantly　suppressed　lattice　thermal　conductivity　of　0.41　W　m(-1)　K-1　at　similar　to　760　K.　A　further　0.5　mol%　CuCl2-doping　triggers　impurity　band　in　the　electronic　structure　of　Bi2S3　and　narrows　the　bandgap　for　optimizing　the　carrier　concentration　at　similar　to　1　x　10(20)　cm(-3),　confirmed　by　both　experimental　results　and　first-principles　density　functional　theory　calculations.　The　optimized　carrier　concentration　and　maintained　low　lattice　thermal　conductivity　give　rise　to　a　high　power　factor　of　similar　to　5.3　mu　W　cm(-1)　K-2　and　high　ZT　that　ranks　as　a　top　value.　This　work　provides　a　new　route　to　achieve　high　thermoelectric　performance　in　n-type　polycrystalline　Bi2S3.