Monoclinic phase Bi6Mo2O15 sub-microwires consisting of MoO4 tetrahedra have been successfully synthesized by a molten salt method. The wide-bandgap sub-microwire exhibits photocatalytic activity toward the photoreduction of CO2 into CH4. The existence of surface oxide vacancies enhanced the photocatalytic activity, which can be easily tuned via different post-heating temperatures, through capturing photo-generated electrons at the surface, thus being beneficial for the separation of electrons and holes and prolonging the lifetime of the electrons.