The discovery of short-wave ultraviolet (SWUV, λ_PM < 266 nm, PM = phase-matching) and deep ultraviolet (DUV, λ_PM < 200 nm) nonlinear optical (NLO) crystals is urgently required and full of challenges. Unlike the conventional strategy of using π-conjugated groups (such as BO₃) as core motifs for constructing UV NLO crystals, herein the long-neglected stereochemically active lone pair (SCALP) groups ClO₃ and BrO₃ are innovatively proposed to be good UV NLO functional motifs based on the group property prediction, and the NLO performance of chlorates and bromates has been investigated systematically for the first time by first-principles methods. Benefiting from the high polarizability of ClO₃ and BrO₃ and their favorable alignments, the halate crystals AClO₃ and ABrO₃ (A = NH₄, K, Rb and Cs) exhibit high SHG coefficients comparable to that of classical β-BaB₂O₄ (4.2–4.8 × KDP for AClO₃ and 6.1–7.1 × KDP for ABrO₃). Meanwhile, their wide band gaps and large optical anisotropy lead to very short λ_PM deep into DUV and SWUV (185–195 nm for AClO₃ and 210–220 nm for ABrO₃). Remarkably, ABrO₃ shows a rare full-wavelength phase-matching capability. Hence AClO₃ and ABrO₃ could be promising DUV and SWUV NLO candidates, respectively, and the UV NLO potential of ClO₃ and BrO₃ is further demonstrated by profound mechanism analysis. This work opens up a new avenue for the development of SWUV and even DUV NLO materials.