A series of beta-AgAl1-xGaxO2 solid-solution materials were explored as novel visible-light-sensitive photocatalysts. These Ag-based solid solutions crystallize in a homogeneous crystal structure with orthorhombic symmetry but possess continuously modulated band gaps from 2.19 to 2.83 eV by decreasing the ratios of Ga/Al. Their photoactivities for iso-propanol degradation were found to be dependent on the variation of chemical compositions. Among them, the beta-AgAl0.6Ga0.4O2 sample showed the highest photocatalytic performance, which simultaneously exhibited 35 and 63 times higher activities than two terminus materials, beta-AgAlO2 and beta-AgGaO2, respectively. The apparent quantum efficiency of this sample for iso-propanol photodegradation achieved up to 37.3% at the wavelength of 425 +/- 12 nm. The theoretical calculation based on density functional theory demonstrated that the levels of valence band maximum of beta-AgAl1-xGaxO2 are similar, but the levels of conduction band minimum are gradually negatively shifted with the increase of the ratio of Ga/Al, thereby continuously narrowing the band gap. Nevertheless, the highest activity observed on beta-AgAl0.6Ga0.4O2 may be attributed to its optimized band structure, which adapts the balance between effective visible-light absorption and adequate redox potentials.