In this study, the adsorption mechanism and hydroxyl radical ((•)OH)-initiated photocatalytic degradation mechanism of styrene onto different (TiO2)n clusters were investigated using density functional theory. Styrene, a typical model atmospheric volatile organic compound (VOC), was found to be readily adsorbed onto (TiO2)n clusters through its vinyl group with strong chemisorption. This suggests that (TiO2)n clusters (sub 1 nm) are able to effectively adsorb and trap styrene. Adsorbed styrene is then easily attacked by (•)OH to form a series of vinyl-OH-adducts. Conversely, phenyl-OH-adducts and H-abstraction products are very difficult to form in this system. Kinetics calculations using canonical variational transition state theory show that temperature has little effect on the rate constants during photocatalytic degradation process. The presence of TiO2 does not change the degradation mechanism of styrene, but can accelerate its photocatalyic degradation rate, and the rate will increase as TiO2 cluster size increases; as such, the TiO2 nano-clusters catalyst should have the photocatalytic ability to effectively degrade styrene. This theory-based study offers insights into the catalytic effect of TiO2 catalyst and the photocatalytic degradation mechanism of benzene series air pollutants at the molecular level.