The　electrocatalytic　conversion　of　CO2　waste　into　liquid　fuels　is　one　of　the　most　promising　approaches　to　contribute　to　carbon　neutrality　and　sustainable　energy　cycles.　Designing　and　screening　efficient　electrocatalysts　to　simultaneously　reduce　the　overpotential　required　for　the　CO2　reduction　reaction　(CO2RR),　enhancing　product　selectivity　and　current　density,　still　remains　challenging.　The　high　concentration　of　S　vacancies　in　sulfides　that　can　serve　as　active　sites　offers　the　possibility　to　address　these　challenges.　Here,　this　study　deciphers　the　positive　roles　of　S　vacancies　in　boosting　the　adsorption　and　activation　of　CO2　by　modulating　the　local　electronic　structure　through　theoretical　simulations　on　the　In4SnS8　platform.　The　comparison　experiments　confirm　that　sulfides　with　a　high　concentration　of　intrinsic　S　vacancies　are　more　competitive　in　selectivity　and　activity.　Afterward,　the　post-processing　heating　desulfurization　method　is　employed　to　create　more　abundant　artificial　S　vacancies　to　further　verify　its　effectiveness.　The　HCOOH　Faradaic　efficiency　(FE)　of　the　post-processed　V-In4SnS8-350　with　both　intrinsic　and　artificial　S　vacancies　is　as　high　as　91%,　which　is　much　higher　than　65%　for　pristine　In4SnS8　with　only　intrinsic　S　vacancies.　Meanwhile,　the　abundant　artificial　S　vacancies　not　only　upgrade　the　HCOOH　yield　from　2.2　to　14.0　mmol·h-1·cm-2　but　also　optimize　the　bias　potential　with　the　highest　FE　from　−1.3　to　−1.0　VRHE.　These　results　highlight　the　effectiveness　triggered　by　intrinsic　and　artificial　S　vacancies　in　enhancing　the　current　density　and　lowering　the　optimal　bias　potential　window,　as　well　as　improving　the　HCOOH　selectivity.　This　work　not　only　provides　a　sound　guideline　for　the　development　of　advanced　sulfide　semiconductor　electrocatalysts　but　also　renders　insightful　information　on　multifunctional　S　vacancies　in　the　CO2RR.　©　2023　American　Chemical　Society.