Ag-based oxides, Ag2M2O7 (M = Mo, W) with a triclinic structure, were prepared using a conventional solid-state reaction method, and their band structures and photocatalytic properties were investigated. From the electronic band structure calculations, Ag2Mo2O7 had an indirect band gap of 2.65 eV, whereas Ag2W2O7 had a direct band gap of 3.18 eV. Moreover, the calculated electron affinities were 4.75 eV for Ag2Mo2O7 and 4.62 eV for Ag2W2O7. The smaller band gap and higher electron affinity of Ag2Mo2O7 than those of Ag2W2O7 originated from the contribution of the unoccupied Mo 4d orbital to the conduction band, which was positioned at a lower energy level than the W 5d orbital. The effects of the different band structure characteristics in Ag2M2O7 (M = Mo, W) on the photocatalytic activity were investigated by the degradation of a Rhodamine B dye solution under visible light irradiation. The results showed that the photocatalytic activity of Ag2Mo2O7 was much higher than that of Ag2W2O7 and comparable with a well-known WO3 powder. The higher photocatalytic property of Ag2Mo2O7 was attributed to its higher optical absorption ability and lower charge recombination rate.