Flexible　Cu2ZnSn(S,Se)(4)　(CZTSSe)　solar　cells　show　great　potential　applications　due　to　low-cost,　nontoxicity,　and　stability.　The　device　performances　under　an　especial　open　circuit　voltage　(V-OC)　are　limited　by　the　defect　recombination　of　CZTSSe/CdS　heterojunction　interface.　We　improve　the　deposition　technique　to　obtain　compact　CdS　layers　without　any　pinholes　for　flexible　CZTSSe　solar　cells　on　Mo　foils.　The　efficiency　of　the　device　is　improved　from　5.7%　to　6.86%　by　highquality　junction　interface.　Furthermore,　aiming　at　the　S　loss　of　CdS　film,　the　S　source　concentration　in　deposition　process　is　investigated　to　passivate　the　defects　and　improve　the　CdS　film　quality.　The　flexible　Mo-foil-based　CZTSSe　solar　cells　are　obtained　to　possess　a　9.05%　efficiency　with　a　V-OC　of　0.44　V　at　an　optimized　S　source　concentration　of　0.68　mol/L.　Systematic　physical　measurements　indicate　that　the　S　source　control　can　effectively　suppress　the　interface　recombination　and　reduce　the　V-OC　deficit.　For　the　CZTSSe　device　bending　characteristics,　the　device　efficiency　is　almost　constant　after　1000　bends,　manifesting　that　the　CZTSSe　device　has　an　excellent　mechanical　flexibility.　The　effective　improvement　strategy　of　CdS　deposition　is　expected　to　provide　a　new　perspective　for　promoting　the　conversion　efficiency　of　CZTSSe　solar　cells.