Nonlinear optical (NLO) materials are the vital components of future photoelectric technologies as they can broaden the tunable wavelength range supplied by common laser sources. However, the necessary prerequisites for a practical NLO material are rather strict. Accordingly, considerable efforts have been focused on finding potential NLO materials. Here we report two asymmetric beryllium-free borates Pna2₁- and P2m-Ba₃Mg₃(BO₃)₃F₃ featuring NLO-favorable ²_∞[Mg₃O₂F₃(BO₃)₂] layered structures. The reversible phase transition among two polymorphs was demonstrated by multiple experimental tests. The optical measurements reveal that Pna2₁-Ba₃Mg₃(BO₃)₃F₃ possesses the optical properties required for ultraviolet NLO applications. Remarkably, Pna2₁-Ba₃Mg₃(BO₃)₃F₃ has a large laser damage threshold, a deep-ultraviolet cutoff edge, a favorable anisotropic thermal expansion as well as the capacity of insolubility in water. These optical properties can be comparable or superior to that of commercial NLO material β-BaB₂O₄, which make Pna2₁-Ba₃Mg₃(BO₃)₃F₃ a promising ultraviolet NLO material.