ZnWO4 catalysts have been successfully synthesized by a hydrothermal crystallization process. ZnWO4 catalyst synthesized at 180 °C for 24 h exhibited relatively high photochemical activity for the decomposition of rhodamine-B, and showed better activity for the degradation of formaldehyde than that of TiO2 (P-25). The generations of the photocurrent and H2 evolution from pure water were also realized. UV bands observed in the catalyst were assigned to the band transition from the occupied O 2p orbitals to the empty W 5d orbitals. On the basis of the analysis of the energy levels and the band structures, the photocatalytic mechanism of the catalyst was also discussed.