The effects of photochemical degradation of soil humic acid on the binding property of benzo(α)pyrene were investigated under solar-simulated irradiations, in which the influence of O2 abundance, irradiation time, and pH were discussed in details. The irradiations substantially decreased the binding coefficient Koc, which positively correlated with the molecular weight and aromaticity of humic acid in the time-course of irradiation. After irradiations, Koc decreased averagely by 33.8% ± 8.3% (determined at the initial pH) and 34.3% ± 4.1% (determined at the post-irradiation pH) respectively in the pH range of 4.1 to 8.0. Increasing O2 abundance accelerated the decrease of the binding coefficient resulted from enhanced photo-degradation of fulvic acids. Compared with the unirradiated sample, the binding coefficient of humic acid-BaP under air-saturated and oxygen-saturated irradiations decreased by 37.2% and 47.1% respectively.