The photolysis ability and adsorption ability of TiO2 was detected and measured using acetaldehyde as a probe. The quantitative relationship between the TiO2 deactivation and the concentration of the surface adsorbed SO42- was obtained by intentional controls of the concentrations of the surface adsorbed SO42-. When the concentration of the surface adsorbed SO42- reached 2.0 wt%, the photolysis ability of the TiO2 photocatalyst decreased by 50%, and when the concentration was 2.9 wt%, the TiO2 adsorption ability decreased by 50%. 90% of the absorbed SO42- can be removed after the samples have been washed by overflowing water for 2 hours at a rate of 2 L.min-1 and the photocatalytic activity can be regenerated. This work will help to predict the lifetime of TiO2 photocatalyst in actual applications as well as to understand its regenerations of the photocatalytic capability.