[en] Silver-containing niobate AgNb₇O₁₈ nanoparticles were prepared via sol-gel polymerized complex synthesis. The sample was given a good XRD Rietveld refinement in a typical perovskite-like structure. The detailed surface properties of AgNb₇O₁₈ nanoparticles were tested by SEM, EDS, TEM, nitrogen adsorption-desorption isotherms and XPS measurements. AgNb₇O₁₈ is characterized via a direct transition and the band energy is 2.76 eV. The hybridization between O 2p and Ag 3d states in valence band contributes to the narrowed band gap. The first principles electronic structure elucidation supports the narrow band gap of AgNb₇O₁₈. The sample shows an efficient degradation of methylene blue dye (MB) solutions. The trapping tests were performed to elucidate the real active species in the photocatalytic reactions. The results indicate that the superoxide radical is the main active species in the photocatalytic system, and the photocatalyst also shows a good stability and durability. - Highlights: • A new semiconductor nanoparticle AgNb₇O₁₈ was prepared. • The silver niobate has a typical perovskite-like structure. • AgNb₇O₁₈ shows an efficient absorption with a narrow gap 2.76 eV. • AgNb₇O₁₈ has an efficient photocatalytic activity on MB photo-degradation. • The photocatalysis mechanism and stabilities were discussed.