Ferrate is a promising oxidizing agent for water treatment. Understanding the reaction characteristics and transformation mechanism of high-valent intermediate irons [Fe(V) and Fe(IV)] remains challenging. Here, we systematically investigated the roles of Fe(VI), Fe(V), and Fe(IV) species for acetaminophen oxidation using reaction kinetics, products, and stoichiometries. Acetaminophen reacts with Fe(VI) via one-electron transfer mechanism, to initiate a sequential conversion process of "Fe(VI)-Fe(V)-Fe(IV)-Fe(III)", with a stoichiometry [Δacetaminophen/Δferrate] up to 2.20:1. The stoichiometry decreased to 1.23:1 after adding pyrophosphate to sequester Fe(V) oxidation, higher than the Fe(VI)-contributed stoichiometry of 0.58:1, indicating the involvement of Fe(IV) species, not inhibited by pyrophosphate. Dimer yields and theoretical calculations demonstrated that the generated phenoxyl radical could reduce Fe(V) into Fe(IV) even in the presence of pyrophosphate, to achieve the sequential one-electron transfer process. For other phenols containing electron-donating substituents, their phenoxyl radicals could also induce the transformation of Fe(V) into Fe(IV). This organic radical-induced conversion could occur in the reaction of ferrate with natural organic matter, and enhance the effective removal of pollutants. This study highlights the interaction of phenoxyl radical with high-valent iron species, and offers new insights to guide future identification of high-valent iron species in ferrate oxidation.