Environmentally persistent free radicals (EPFRs) have been widely detected in superfund sites and atmospheric particles contaminated with organic contaminants, but the impacts of environmental factors such as light irradiation on the formation and evolution of EPFRs remain unclear. In the present study, in-situ irradiated Fourier transform infrared spectrometer and electron paramagnetic resonance were applied to probe the formation mechanisms of EPFRs during photo-transformation of polycyclic aromatic hydrocarbons (PAHs) on montmorillonite surface. EPFRs were only detected on Fe(III)-montmorillonite containing PAHs with relatively high electron-donating ability, such as anthracene (ANT), but not in the systems of Fe(III)-montmorillonite spiked with phenanthrene or Na(I)-montmorillonite. The 1/e lifetime of the EPFRs was much shorter under light irradiation (5.49 h) than in dark (30.3 h), suggesting that light irradiation facilitated the transformation of EPFRs. On the one hand, light irradiation promoted direct electron transfer from ANT to the mineral surface, accelerating the formation of PAHs-type radical cations. On the other hand, light irradiation induced the generation of reactive oxygen species, which facilitated the transformation from radical cations to oxygenic EPFRs, which finally led to ANT degradation. This work clarified the underlying mechanisms for EPFRs generation and evolution on clay minerals.