Herein, a new fluorooxoborate α-KB 3 O 4 F 2 is synthesized successfully in a closed system. It crystallizes in the P2 1 /n space group and features the rare one-dimensional (1D) zigzag [B 6 O 8 F 4 ] ∞ chains built by fundamental building blocks (FBBs) [B 6 O 9 F 4 ]. To the best of our knowledge, KB 3 O 4 F 2 is a unique example of inorganic anhydrous borate whose two polymorphs show 1D B−O/F chains constructed by different FBBs but the same symmetry operation. Different from β-KB 3 O 4 F 2 , the inconsistent arrangement of π-conjugated [B 2 O 5 ] units in α-KB 3 O 4 F 2 makes it exhibit a small birefringence (0.011@546 nm). Further, α-KB 3 O 4 F 2 owns a deep-ultraviolet (DUV) cutoff edge (<200 nm), suggesting that it has a potential application as the zero-order waveplate material in the DUV region.