Exploring significant ultraviolet/deep‐ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase‐matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue‐shift the PM wavelength. Here, the introduction of a fluorinated tetrahedral boron‐centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, the BF4− unit with a large HOMO–LUMO band gap was introduced to the Na−B−O−F system and Na4B8O9F10 was designed and synthesized successfully for the first time. Na4B8O9F10 with an optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (0.036@1064 nm). Notably, Na4B8O9F10 is the first acentric crystal with BF4− units among the reported metal–fluorooxoborate systems, involving isolated BF4− and novel B7O10F65− fundamental building blocks. Na4B8O9F10, the first non‐centrosymmetric crystal with BF4− units among the reported metal‐fluorooxoborate systems, was designed and synthesized. It features isolated BF4− groups and novel B7O10F65− fundamental building blocks, and achieves an unexpected short λPM of 240 nm induced by optimized chromatic dispersion with such a small birefringence (0.036@1064 nm).