Titanium dioxide/tungsten disulfide (TiO2/WS2) composite photocatalysts were fabricated via a one-step hydrothermal synthesis process, using TiCl4 as titanium source and bulk WS2 as sensitizer. The morphology, structure, specific surface area and optical absorption properties of the composite photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), specific surface area analyzer and ultraviolet-visible diffuse reflection spectrum (UV-vis DRS), respectively. The photocatalytic activity of as-prepared photocatalysts was evaluated by the degradation of methyl orange (MO) under illumination of 500W Xenon lamp. The results indicated that TiO2/WS2 composite photocatalysts possessed excellent photocatalytic activity, and ~95% of the degradation rate for MO was reached when molar ratio of WS2 to TiO2 was 0.004 and the irradiation time was 60 min. Moreover, the carrier trapping experiment and fluorescence spectra showed that ·O2- was the key component in the photocatalytic degradation process and O2 was reduced to be ·O2- by the electrons from the conduction band of TiO2 and WS2 for the degradation of MO.