A　favorable　back　contact　interface　is　essential　for　obtaining　efficient　flexible　Cu2ZnSn(S,　Se)4　(CZTSSe)　thin　film　solar　cells.　Here,　Mo/CZTSSe　back　interfaces　are　improved　by　treating　Mo　foil　substrates　with　a　plasma　method.　MoO3　layers　are　generated　on　the　Mo　foils　substrates　with　the　plasma　treatment,　enhancing　the　wettability　of　the　substrate　surfaces　and　thus　promoting　the　grain　growth　of　the　CZTSSe　films.　The　contact　resistance　and　barrier　height　(Phi　B)　at　the　Mo/CZTSSe　interface　is　decreased,　enhancing　the　transport　of　the　hole　carrier.　Moreover,　the　quality　of　the　CZTSSe/CdS　heterojunction　is　significantly　improved　owing　to　the　modified　CZTSSe　absorber.　Ultimately,　the　flexible　CZTSSe　solar　cell　has　achieved　a　power　conversion　efficiency　(PCE)　of　10.59　%　without　antireflection　coating.　Benefiting　from　the　positive　modification　at　Mo/CZTSSe　interface,　the　PCE　of　the　flexible　device　still　maintains　95.87　%　of　the　initial　PCE　after　bending　5000　times　at　the　bending　diameter　of　2　cm.　This　simple　and　effective　way　of　optimizing　the　back　interface　provides　a　new　direction　for　the　PCE　improvement　of　the　flexible　CZTSSe　solar　cells.