In this paper, a series of purephase Bi2Fe4O9 nanocrystals were synthesized by adding different content of FeCl2 in the process of hydrothermal method, and the lower valence iron ions were successfully introduced into Bi2Fe4O9. The UVvisible absorption spectrum data proves that the addition of FeCl2 reduces the optical band gap of Bi2Fe4O9 from 2.06 eV to 1.96 eV. Both the scanning electron microscope images and Xray diffraction patterns prove that the synthesized samples are purephase square flakes Bi2Fe4O9 nanocrystals. Three synthesized Bi2Fe4O9 nanocrystal samples (S1, S2, S3, the FeCl2 content of the three samples increased from 0 during the synthesis process, corresponding to x=0, 0.2 mmol, 0.4 mmol, respectively) were used for photocatalytic degradation of methyl orange solutions under a 500 W mercury lamp irradiation. It is found that the photocatalytic degradation rate of the three samples is the fastest when 20 μL of hydrogen peroxide is added, and the photocatalytic degradation rate of the S3 sample is faster than that of S1 and S2. In the case of appropriate addition of hydrogen peroxide, the addition of FeCl2 can improve the performance of Bi2Fe4O9 in photocatalytic degradation of dye molecules.