Pure perovskite BiFeO3, sillenite Bi2Fe4O9, and BiFeO3/Bi2Fe4O9 were synthesized facilely by controlling the precursor Bi/Fe ion ratio through a hydrothermal method. The phase composition, morphology, optical properties, and photocatalytic activity of as-prepared samples were investigated in detail. Our results indicate that the morphology and properties of products strongly dependent on the precursor Bi/Fe ion ratio. Photocatalysis of Congo Red reveals that sillenite Bi2Fe4O9 shows superior activity to perovskite BiFeO3, and BiFeO3/Bi2Fe4O9 exhibited higher activity with 71.45% degradation rate in 90 min. It provides an easy and efficient way to tune the composition and photocatalytic activity of Bi-based oxides.