A large band gap is an important prerequisite for deep-ultraviolet materials. Here we provide a strategy to adjust the band gaps by tuning the F/B ratios. By analyzing the molecular orbitals of fluorooxo-functional groups and the electronic structures of fluorooxoborates, we find that the fluorooxo-functional groups with high F/B ratio change the composition of the highest occupied and lowest unoccupied molecular orbitals and the corresponding valence-band maximum/conduction-band minimum.