Steel pickling sludge provides abundant iron resource for the preparation of bimetallic catalyst used in heterogeneous advanced oxidation processes (AOPs). In this study, steel pickling sludge was used to synthesize CoFe₂O₄ layered catalyst through a convenient one-step solvothermal reaction and applied as a powerful peroxymonosulfate (PMS) activator to decolorize tartrazine solution. The layered microstructure, composition, and structure of the as-prepared CoFe₂O₄ catalyst were characterized by scanning electron microscope, X-ray photoelectron spectroscopy, and X-ray diffraction. The influences of catalyst dosage, PMS dosage, and initial solution pH on the decolorization efficiency were systematic investigated and optimized, the results showed that CoFe₂O₄ layered catalyst exhibited excellent catalytic effect with 96% decolorization efficiency in 15 min. SO₄²⁻, Cl⁻, and NO₃⁻ had little effect on the decolorization efficiencies compared to HCO₃⁻ and CO₃²⁻. Both free radicals (∙SO₄⁻, ∙OH, and ∙O₂⁻) and non-radicals (¹O₂ and mediated electron transfer) pathways coexist in CoFe₂O₄ layered catalyst/PMS system, and ∙SO₄⁻ was the dominate reactive oxygen species. Based on DFT calculation, the plausible decolorization mechanism of tartrazine in CoFe₂O₄ layered catalyst/PMS system could be well elucidated. This study offers an economic reuse strategy to convert the “steel pickling waste sludge” to low-cost and higher-value bimetallic catalyst for the practical application in AOPs.