Abstract: Aiming at improving the understanding of occurrence mechanism and affecting factors of induction period, a small laboratory-made reactor with batch kinetic test method was adopted to investigate the induction period occurrence reasons and the key affecting factors during phenol oxidation by H₂O₂ over clay-based catalysts. The results showed that the contact of active metals with H₂O₂ and the sequent surface modification, by decreasing pH via the reduction of ≡Fe (III) to ≡Fe (II), jointly determined the induction period. A Fermi’s equation could fit well the kinetic process (R² > 0.99), and the reaction temperature and initial solution pH strongly affected the induction time (tI) which reduced from 59 min to 22 min and from 129 min to 0 min (no induction period), respectively, with increasing temperature and decreasing pH. The catalyst amount and H₂O₂ concentration had less effect on tI which reduced from 69 to 32 min and from 75 to 52 min, respectively, with increasing the contents of catalyst and H₂O₂. Although the undesirable induction time could be shortened or eliminated by adjusting reaction parameters, considering its cost and environmental risk, the following optimum reaction conditions should be adopted for CWPO of actual phenol-containing wastewater over clay-based catalysts: the mole ratio of H₂O₂ to phenol being 15, catalyst concentration being 0.8-1.0 g/L, temperature being 30-40 °C, pH of matrix not adjusted, and reaction time being 2 h.