Photocatalytic degradation of sulfanilamide,sulfamethoxazole,sulfathiazole and sulfadimidine in aqueous suspension was investigated by using carbon nanotube modified TiO2 nanocomposite as the photocatalyst under visible-light and UV-light irradiation.The correlativity between molecule electronic structure and photocatalytic activity of sulfonamides was investigated by using quantum chemical method (density functional theory) DFT/B3LYP via calculating electronic structure changes.The results showed that the photocatalytic degradation rate of the four sulfonamides under visible-light irradiation was in the order of sulfanilamide>sulfathiazole>sulfamethoxazole>sulfadimidine.The photocatalytic activity of sulfonamides could be described better by their electronic structures:increasing the dipole moment and energy of HOMO and decreasing the n-octanol-water partition coefficient and the HOMO-LUMO gap lead to an increase in the photocatalytic degradation rate.The photocatalytic degradation rates of the sulfonamides in mixture relate to not only their molecule electronic structures but also the adsorption on the photocatalyst.