Double perovskite Ba2FeNbO6 (BFN) powders were synthesized by molten salt method. Their optical properties were characterized by UV-Vis absorption spectra, and the band gaps (Eg) were determined to be 2.12 - 2.25 eV. The Eg values were tuned by adjusting the processing parameters of molten-salt route (e.g., annealing temperature, holding time as well as the molten salt ratios). The varied Eg values are ascribed to the different content ratios of Fe 3+ to Fe 2+ ions and the oxygen vacancies in the BFN powders. Oxygen vacancies in the BFN powders result in the distortions of FeO6 octahedrons, leading to different Fe-O bond lengths. Thus, the overlapping between the O-2p < /em> and Fe-3d orbitals is changed, making the Eg values changed. A small absorption shoulder with absorption edge at 650 nm (Eg ~ 1.91 eV) observed in the BFN powders, was ascribed to the d-d electronic transition from the Fe 3d-b2g to Fe 3d-b1g orbitals, which were formed by further splitting of Fe 3d orbitals in the distorted octahedral field. The present work offers an effective approach to tuning the Eg of BFN powders, which find promising applications in the fields of photovoltaic and photocatalytic devices.