Discovering　new　deep　ultraviolet　(DUV)　nonlinear　optical　(NLO)　materials　is　the　current　research　hotspot.　However,　how　to　perfectly　integrate　several　stringent　performances　into　a　crystal　is　a　great　challenge　because　of　the　natural　incompatibility　among　them,　particularly　wide　band　gap　and　large　NLO　coefficient.　To　tackle　the　challenge,　a　boron-rich　closed-loop　strategy　is　supposed,　based　on　which　a　new　barium　borate,　Ba4B14O25,　is　designed　and　synthesized　successfully　via　the　high-temperature　solid-state　melting　method.　It　features　a　highly　polymeric　3D　geometry　with　the　closed-loop　anionic　framework　[B14O25]8−　constructed　by　the　fundamental　building　blocks　[B14O33]24−.　The　high-density　π-conjugated　[BO3]3−　groups　and　the　fully　closed-loop　B–O–B　connections　make　Ba4B14O25　possess　excellent　NLO　properties,　including　short　UV　cutoff　edge　(＜200 nm),　large　second　harmonic　generation　response　(3.0　×　KDP)　and　phase-matching　capability,　being　a　promising　DUV-transparent　NLO　candidate　material.　The　work　provides　a　creative　design　strategy　for　the　exploration　of　DUV　NLO　crystals.　©　2023　Wiley-VCH　GmbH.