Abstract: The measurements of positron annihilation lifetime spectrum for chlorophyll in cyclohexane (77 K) and in alcohol (296 K) were carried out under illumination at different light intensity. A photomagnetic positronium annihilation model was proposed to explain the experimental facts. The changes of annihilation parameters I_2 and Λ_2 for frozen chlorophyll cyclohexan solution at 77 K with relative light intensity suggest that the photomagnetic quenching effect of positronium can serve to trace the concentration changes of photo-excited triplet states in photosensitive material.