Polarization modulation of deep‐UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double‐modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep‐UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH₄)₄[B₁₂O₁₆F₄(OH)₄] with giant cluster as a deep‐UV birefringent crystal with large birefringence (Δn_exp.=0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near‐plane arrangement of [B₁₂O₁₆F₄(OH)₄] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π‐conjugated [BO₃] and [BO₂OH] units are the main source of this large optical anisotropy.