Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in soils, and their inherent toxicity and persistence pose serious threats to terrestrial animals and plants. This study reports the synthesis of a biochar-supported Fe₂O₃/CoFe₂O₄ nanocomposite (Fe₂O₃/CoFe₂O₄@BC) as a peroxymonosulfate (PMS) activator for the degradation of PAHs in contaminated soil. The one-step prepared Fe₂O₃/CoFe₂O₄@BC exhibits abundant nanoporous structures, with a specific surface area of 158.34 m² g⁻¹ and average pore size of 2.84 nm. Compared with pristine biochar (BC), Fe₂O₃/CoFe₂O₄@BC shows superior catalytic performance in activating PMS for the efficient degradation of seven kinds of 3–4 ring PAHs in soil. Based on electron paramagnetic resonance (EPR) analysis, electrochemical analysis and scavenger quenching experiments, the catalytic mechanism was elucidated, revealing that ^•OH, SO₄^•−, and electron transfer were primarily responsible for the degradation of phenanthrene (PHE) in soil. The presence of coexisting soil constituents, including Cl⁻, HCO₃⁻, NO₃⁻ and humic acid (HA), was found to inhibit PHE degradation. Furthermore, plant growth experiments revealed that the Fe₂O₃/CoFe₂O₄@BC/PMS system effectively alleviated the ecotoxicity of PHE-contaminated soil. This work thus provides a promising nanocatalyst for PMS-mediated remediation of soil contaminated with refractory organic pollutants and offers a feasible strategy for the in situ chemical remediation of PAH-polluted soil.