In　this　manuscript,　we　report　the　simulation　of　electronic　structures　of　TiO2　doped　with　Ag　using　first-principles　calculation,　based　on　the　density　functional　theory　(DFT)　The　mechanism　to　improve　the　visible-light　photocatalytic　activity　for　TiO2　doped　with　Ag.　EF　was　displaced　into　the　valence　band　(VB),　narrowing　the　band　gap.　An　independent　energy　band　was　also　introduced　above　the　VB　and　an　energy　band　with　the　width　of　0.9　eV　is　introduced　under　the　conduction　band　(CB),　also　resulting　in　the　narrowing　of　the　band　gap.　New　VB　and　CB　were　formed　by　three　orbitals　O-2p,　Ti-3d　and　Ag-4d.　Composite　TiO2　nanotubes　with　Ag-doping　were　synthesized　using　the　alkaline　fusion　and　hydrothermal　method　under　ambient　atmosphere.　XRD　results　indicated　composite　Ag-TiO2　nanotubes　ranging　from　the　single　anatase　phase　to　the　anatase　and　rutile　mixed　phase.　XPS　showed　an　increase　in　the　surface　acidity　and　photo　catalytic　activity　of　TiO2　nanotubes　due　to　the　presence　of　OH　radicals.　Furthermore　Ag-doping　also　facilitated　the　separation　of　photo-holes　and　photo-electrons　and　enhanced　the　optical　properties.