Deep-ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two-dimensional (2D) crystal structure prediction method combined with the first-principles calculations, here we propose layered 18-membered-ring (18MR) boron oxide B2O3 polymorphs as high-performance NLO materials. 18MR-B2O3 with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR-B2O3AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V-1, the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe2BO3F2 and comparable to that of beta-BaB2O4. Its unusually large birefringence of 0.196 at 400 nm guarantees the phase-matching wavelength lambda(PM) to reach this material's extreme absorption edge of approximate to 154 nm.