The　severe　carrier　recombination　at　the　CZTSSe/CdS　heterojunction　interface　and　high　toxicity　of　Cd　limit　the　further　development　of　flexible　Cu2ZnSn(S,　Se)4　(CZTSSe)　solar　cells.　Here,　we　apply　a　suitable　and　environmentally　friendly　Zn1-xSnxO　(ZTO)　buffer　layer　to　suppress　the　interface　recombination　in　the　flexible　solar　cell　for　the　first　time.　The　ZTO　buffer　layers　with　good　qualities　have　been　successfully　obtained　by　varying　the　Sn/(Zn　+　Sn)　value.　The　solar　cell　has　obtained　an　efficiency　of　8.7%,　which　is　the　highest　efficiency　reported　for　a　Cd-free　flexible　CZTSSe　solar　cell　so　far.　The　optimal　device　has　more　ideal　diode　parameter　values,　indicating　that　the　interface　quality　is　significantly　improved.　The　VOC-T　characteristic　demonstrates　that　the　interfacial　recombination　of　flexible　ZTO　devices　has　been　greatly　reduced.　The　theoretical　simulations　show　that　the　flexible　CZTSSe　devices　with　optimal　ZTO　layers　achieve　good　band　alignment　to　reduce　the　interface　recombination,　elucidating　the　intrinsic　link　between　the　band　structure　and　the　device　performance.　This　investigation　provides　a　solution　to　achieve　environmentally　friendly　and　efficient　flexible　CZTSSe　solar　cells　by　designing　the　interface　engineering　of　the　absorber/buffer　heterojunction　in　solar　cells.