As　one　of　the　leading　photovoltaic　materials,　Cu(In,Ga)(Se,S)(2)　has　two　distinct　advantages　over　other　candidates.　First,　its　conduction　band　and　valence　band　could　be　independently　tuned　by　adjusting　the　Ga/(Ga　+　In)　and　S/(S　+　Se),　respectively.　Next,　its　composition　distribution　could　be　feasibly　tuned　by　controlling　the　thin　film　deposition　process.　These　merits　have　been　utilized　in　fabricating　the　Cu(In,Ga)(Se,S)(2)　solar　cells　with　world　record　efficiency.　However,　these　features　have　not　yet　been　satisfactorily　simulated.　In　order　to　understand　the　mechanism　of　high-performance　Cu(In,Ga)(Se,S)(2)　solar　cells,　we　obtained　the　tunable　bandgap　of　absorber　layer　and　suitable　conduction　band　offset　between　absorber　layer　and　buffer　layer　via　Ga　and　S　concentration　and　gradient,　by　combining　both　first-principle　calculations　and　numerical　simulations　with　Poisson　equation　and　the　continuity　equation.　In　particular,　compared　with　other　photovoltaic　materials,　the　bandgap　of　Cu(In,Ga)(Se,　S)(2)　could　be　tuned　flexibly　across　the　layer　thickness　to　realize　spatial　energy　band　optimization　with　doubleconcentration-grading　absorber.　The　efficiency　of　23.29%　was　obtained　based　on　theoretical　calculations　and　numerical　simulations,　which　is　almost　the　same　as　the　reported　result,　in　the　considerations　of　(i)　optimum　bandgap　of　absorber,　(ii)　band　energy　offset,　(iii)　double　(Ga-　and　S-)　concentration　grading,　and　(iv)　low　defect　density　in　the　absorber　layer.　In　addition,　we　obtained　a　Cu(In,Ga)(Se,S)(2)　solar　cell　with　a　high　S　content　uniform　band　gap　efficiency　of　23.53%.　Therefore,　we　provided　an　understanding　for　tunable　engineered　band　energy　research　in　high-efficient　Cu(In,Ga)(Se,S)(2)　solar　cells.