Cation　substitution　plays　a　crucial　role　in　improving　the　efficiency　of　Cu2ZnSn(S,Se)(4)　(CZTSSe)　solar　cells.　In　this　work,　we　report　a　significant　efficiency　enhancement　of　flexible　CZTSSe　solar　cells　on　Mo　foils　by　partial　substitution　of　Cu+　with　Ag+.　It　is　found　that　the　band　gap　(E-g)　of　(Cu1-xAgx)(2)ZnSn(S,Se)(4)　(CAZTSSe)　thin　films　can　be　adjusted　by　doping　with　Ag　with　x　from　0　to　6%,　and　the　minimum　E-g　is　achieved　with　x　=　5%.　We　also　found　that　Ag　doping　can　obviously　increase　the　average　grain　size　of　the　CAZTSSe　absorber　from　0.4　to　1.1　mm.　Additionally,　the　depletion　width　(W-d)　at　the　heterojunction　interface　of　CAZTSSe/CdS　is　found　to　be　improved.　As　a　result,　the　open-circuit　voltage　deficit　(V-oc,V-def)　is　gradually　decreased,　and　the　band　tailing　is　suppressed.　Benefiting　from　the　enhanced　open-circuit　voltage　(V-oc),　the　power　conversion　efficiency　(PCE)　is　successfully　enhanced　from　4.34%　(x　=　0)　to　6.24%　(x　=　4%),　and　the　V-oc,　def　decreases　from　915　to　848　mV.