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 Pna2(1)- and P (6) over bar 2m-Ba3Mg3(BO3)(3)F-3 featuring NLO-favorable (2)(infinity)[Mg3O2F3(BO3)(2)] layered structures. The reversible phase transition among two polymorphs was demonstrated by multiple experimental tests. The optical measurements reveal that Pna2(1)-Ba3Mg3(BO3)(3)F-3 possesses the optical properties required for ultraviolet NLO applications. Remarkably, Pna2(1)-Ba(3)Mg3(BO3)(3)F-3 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 beta-BaB2O4, which make Pna2(1)-Ba3Mg3(BO3)(3)F-3 a promising ultraviolet NLO material.