Focusing　on　the　low　open　circuit　voltage　(V-OC)　and　fill　factor　(FF)　in　flexible　Cu2ZnSn(S,Se)(4)　(CZTSSe)　solar　cells,　indium　(In)　ions　are　introduced　into　the　CZTSSe　absorbers　near　Mo　foils　to　modify　the　back　interface　and　passivate　deep　level　defects　in　CZTSSe　bulk　concurrently　for　improving　the　performance　of　flexible　device.　The　results　show　that　In　doping　effectively　inhibits　the　formation　of　secondary　phase　(Cu(S,Se)(2))　and　V-Sn　defects.　Further　studies　demonstrate　that　the　barrier　height　at　the　back　interface　is　decreased　and　the　deep　level　defects　(Cu-Sn　defects)　in　CZTSSe　bulk　are　passivated.　Moreover,　the　carrier　concentration　is　increased　and　the　V-OC　deficit　(V-OC,V-def)　is　decreased　significantly　due　to　In　doping.　Finally,　the　flexible　CZTSSe　solar　cell　with　10.01%　power　conversion　efficiency　(PCE)　has　been　obtained.　The　synergistic　strategy　of　interface　modification　and　bulk　defects　passivation　through　In　incorporation　provides　a　new　thought　for　the　fabrication　of　efficient　flexible　kesterite-based　solar　cells.