BiOCl/Bi 2 Fe 4 O 9 photocatalyst was prepared by a coprecipitation−hydrothermal method. The heterojunction structure generated by the composite of BiOCl and Bi 2 Fe 4 O 9 reduced the electron−hole recombination efficiency and improved the degradation rate of RhB. At 240 min, 20% BiOCl/Bi 2 Fe 4 O 9 represented the excellent degradation effect on 10 mg/L RhB; the degradation efficiency reached 99.56%; and the reaction rate constant was 0.01534 min −1 , which was 5.76 times and 6.06 times that of Bi 2 Fe 4 O 9 and BiOCl, respectively. The main active substance of the photocatalytic degradation of dyes was superoxide radical O 2 − •. Five cycles of the experiment proved the relative stability of BiOCl/Bi 2 Fe 4 O 9 .