In order to improve the photoactivity, many attempts have focused on increasing the exposure of highly reactive surfaces on crystals. However, the connection between the reactive surfaces and enhancement is still elusive. Herein, Bi2WO6 nanostructured bipyramids with a large fraction of {100} facets are fabricated by the solvothermal method. The formation of "Bi-O" dimer vacancy pairs on the {100} high-energy facets is responsible for the reduction in band gap and the decrease in the recombination of photo-excited charge carriers, which is unambiguously confirmed by the positron annihilation spectra (PAS), X-ray photoelectron spectrum (XPS), and theoretical calculations. The effective separation of electron-hole pairs and the narrowing bandgap significantly improve the photoactivity of Bi2WO6 nanobipyramids, especially under solar light irradiation. These findings can be applied broadly to the design and fabrication of energy efficient and robust catalysts.