Despite　the　pivotal　role　of　stannum　doping　in　achieving　ultrahigh　piezoelectric　performance　in　barium　titanate-based　ceramics,　the　fundamental　mechanisms　underlying　this　enhancement　remain　elusive.　Here,　we　introduce　a　single　variable　nonstoichiometric　stannum　strategy　in　lead-free　barium　titanate-based　ceramics　with　giant　piezoelectricity,　revealing　that　stannum　doping　contributes　intrinsically　and　extrinsically　to　enhance　piezoelectricity.　Density　functional　theory　calculations　elucidate　the　intrinsic　enhancement　of　polarization　arising　from　lattice　distortion　and　increased　space　for　titanium-oxygen　bonds　induced　by　optimal　stannum　doping,　which　is　corroborated　by　Rayleigh　analysis.　A　phase　transition　from　ferroelectric　multiphase　coexistence　to　paraelectric　phase　is　observed,　alongside　a　rapid　miniaturized　and　eventually　disappeared　domains　with　increasing　stannum　doping.　This　evolution　in　phase　structure　and　domain　configuration　induces　a　nearly　vanishing　polarization　anisotropy　and　low　domain　wall　energy,　facilitating　easy　polarization　rotation　and　domain　wall　motion,　thereby　significantly　contributing　to　the　extrinsic　piezoelectric　response.　Consequently,　the　origins　of　ultrahigh　performance　can　be　attributed　to　the　synergistic　effect　of　stannum-induced　intrinsic　and　extrinsic　contributions　in　barium　titanate-based　ceramics.　This　study　provides　fundamental　insights　into　the　role　of　doping　elements　and　offers　guidance　for　the　design　of　high-performance　piezoelectrics.