Fe3O4(named as Fe3O4-PR) was prepared by the precursor method from FeSO4 ,a byproduct of titanium dioxide, and it was used as a catalyst to catalyze the degradation of active brilliant red X-3B by H2O2. The effects of pH, H2O2 dosage and Fe3O4-PR dosage on the catalytic degradation reaction were investigated. And it was explored that the catalytic activity and stability of Fe3O4-PR. Finally, the mechanism of the catalytic degradation reaction was investigated. XRD, FT-IR, SEM and particle size analysis showed that the multi-faceted three-dimensional structure Fe3O4 with an average particle size of 410 nm could be successfully prepared by the precursor method. Under the conditions of pH=3, H2O2 dosage of 3 mM and Fe3O4-PR dosage of 1g/L, the degradation rate of reactive brilliant red X-3B was 97%, and the TOC removal rate was 47%. Fe3O4-PR not only has good stability, but also has better catalytic activity than Fe3O4(namedasFe3O4-CO) prepared by traditional chemical coprecipitation. The mechanism investigation shows that the catalytic degradation reaction occurs at the active site on the catalyst surface, and the redox cycle exists in the degradation process. ?OH is the main active material.