The　electrochemical　reduction　of　CO2　to　formate　is　an　effective　approach　to　achieving　carbon　neutrality.　However,　the　instability　of　catalyst　remains　a　significant　limitation.　Doping　heteroatoms　is　a　recognized　method　for　tuning　the　coordination　environment　to　enhance　both　electrochemical　performance　and　stability.　Herein,　we　report　a　facile　and　mild　hydrothermal　method　employing　sulfur　doping　to　tune　the　coordination　environment　of　bismuth　active　sites.　Sulfur　doping　modifies　the　coordination　environment　of　active　sites　by　partially　substituting　O　atoms　bonded　with　bismuth　active　sites　and　simultaneously　creating　an　abundance　of　oxygen　vacancies,　which　effectively　stabilize　the　active　sites.　This　results　in　notably　improved　stability,　reaching　110　h　with　a　significant　FEHCOOH　(around　100　%).　The　bismuth-based　catalyst　modified　with　sulfur　exhibits　an　impressive　FEHCOOH　over　95　%　across　a　wide　applied　potential　range　of　-0.7　to　-1.4　VRHE.　The　flower-like　structure　increases　the　electrochemical　surface　area,　promoting　greater　exposure　of　bismuth　active　sites.　This　leads　to　a　high　formate　production　rate　of　1248　mmol　center　dot　h-1　center　dot　cm-2　for　the　Cu-BiS　catalyst,　which　is　1.7　times　that　of　the　Cu-Bi　catalyst.　This　strategy　provides　an　effective　way　to　tune　the　coordination　environment　of　active　sites　and　stabilize　them.