The invention discloses a carbon-modified silver phosphate / titanoniobate compound visible light pholocatalyst and a preparation thereof. The preparation method comprises the following specific steps: mixing and grinding Nb2O5, TiO2 and K2CO3, then heating to obtain KTiNbO5 powder; adding KTiNbO5 powder to four methyl ammonium bromide solution and stirring to obtain a modified KTiNbO5 suspension; dissolving ammonium carbamate in methanol at the temperature of 20-25 DGE C and adding silver oxide to obtain the organic complexes of silver; adding the organic complexes of silver to the modified KTiNbO5 suspension, stirring, separating, drying, and calcining in a vacuum state, keeping for 2-3 hours, adding a small amount of air, taking out after natural cooling, grinding, adding into a phosphoric acid solution, depositing, separating, and drying to obtain a carbon-modified silver phosphate / titanoniobate compound visible light pholocatalyst. The preparation method disclosed by the invention has the advantages that silver and carbon are connected directly and tightly, the modification effect is relatively good, and a plurality of neterogeny semiconductor doped materials are adopted, and the catalytic ability is greatly improved.