A new member of fluorooxoborates, Li2Na0.9K0.1B5O8F2, was obtained in the sealed system, and single-crystal X-ray diffraction was used to determine its structure. It contains a three-dimensional framework stacking of [B5O8F2](3-) layers extending into the ac plane. Detailed structural comparisons among all of the fluorine-containing alkali -metal borates suggest that the [B5O8F2](3-) layer composed of the new fundamental building blocks B5O8F2 represents a new structure type of fluorooxoborate. The IR spectrum verifies its structural validity. The deep-ultraviolet spectral meastremen shows that it has no obvious absorption in the range of 180-300 nm, and its cutoff edge is under 180 nm. In addition, theoretical calcillations were done to help us understand its electronic structure and optical properties.