Two kinds of surface-modified TiO2 photocatalysts, PEG-TiO2 nanocrystals and Ba-TiO2 nanocrystals, are prepared and characterized by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD) analysis. The modified TiO2 nanocrystals are small particles with irregular shape and size below 10 nm. Reactive Orange 72, with a -N=N- group, is selected as a model dye. The photocatalytic activities of the modified TiO2 nanocrystals for redox reactions of the azo group under UV irradiation are compared and the mechanism of the reduction of the dye photocatalyst is discussed. The results show that the photocatalysts can efficiently degrade Reactive Orange 72 (50 mg/L) within 2 min. The dye is completely broken down into small molecular compounds. The Ba-TiO2 nanocrystals have oxygen vacancies that serve as internal SEDs, and shows stronger photocatalytic reactivity than that of the unmodified TiO2 nanocrystal and PEG-TiO2. The Ba-TiO2 nanocrystals show the fastest decolorization rate and the best maintained performance for five cycles. The surface-modified TiO2 nanocrystals have potential application in the environmental, photochemical, and photovoltaics fields.